encoding-profile: Add a way to specify a factory-name when specifying caps

In some cases you want to ensure that a specific element factory is used
while requiring some specific caps but this was not possible. You can
now do `qtmux:video/x-prores,variant=standard|factory-name=avenc_prores_ks`
to ensure that the `avenc_prores_ks` factory is used to produce the
'standard' variant of prores video stream.

This also enhances a bit the documentation

Part-of: <https://gitlab.freedesktop.org/gstreamer/gstreamer/-/merge_requests/6875>

girs/GstPbutils-1.0.gir

@@ -2116,9 +2116,26 @@ container profile. If 0, it is not a mandatory stream</doc>
         </parameters>
       </method>
       <method name="set_preset" c:identifier="gst_encoding_profile_set_preset">
-        <doc xml:space="preserve" filename="../subprojects/gst-plugins-base/gst-libs/gst/pbutils/encoding-profile.c">Sets the name of the #GstElement that implements the #GstPreset interface
+        <doc xml:space="preserve" filename="../subprojects/gst-plugins-base/gst-libs/gst/pbutils/encoding-profile.c">Sets the name of the preset to be used in the profile.
-to use for the profile.
+This is the name that has been set when saving the preset.
-This is the name that has been set when saving the preset.</doc>
+You can list the available presets for a specific element factory
+using  `$ gst-inspect-1.0 element-factory-name`, for example for
+`x264enc`:
+
+``` bash
+$ gst-inspect-1.0 x264enc
+...
+Presets:
+ "Profile Baseline": Baseline Profile
+ "Profile High": High Profile
+ "Profile Main": Main Profile
+ "Profile YouTube": YouTube recommended settings (https://support.google.com/youtube/answer/1722171)
+ "Quality High": High quality
+ "Quality Low": Low quality
+ "Quality Normal": Normal quality
+ "Zero Latency"
+```
+ }</doc>
         <source-position filename="../subprojects/gst-plugins-base/gst-libs/gst/pbutils/encoding-profile.h"/>
         <return-value transfer-ownership="none">
           <type name="none" c:type="void"/>
@@ -2135,7 +2152,9 @@ This is the name that has been set when saving the preset.</doc>
         </parameters>
       </method>
       <method name="set_preset_name" c:identifier="gst_encoding_profile_set_preset_name">
-        <doc xml:space="preserve" filename="../subprojects/gst-plugins-base/gst-libs/gst/pbutils/encoding-profile.c">Sets the name of the #GstPreset's factory to be used in the profile.</doc>
+        <doc xml:space="preserve" filename="../subprojects/gst-plugins-base/gst-libs/gst/pbutils/encoding-profile.c">Sets the name of the #GstPreset's factory to be used in the profile. This
+is the name of the **element factory** that implements the #GstPreset interface not
+the name of the preset itself (see #gst_encoding_profile_set_preset).</doc>
         <source-position filename="../subprojects/gst-plugins-base/gst-libs/gst/pbutils/encoding-profile.h"/>
         <return-value transfer-ownership="none">
           <type name="none" c:type="void"/>
@@ -2146,7 +2165,7 @@ This is the name that has been set when saving the preset.</doc>
             <type name="EncodingProfile" c:type="GstEncodingProfile*"/>
           </instance-parameter>
           <parameter name="preset_name" transfer-ownership="none" nullable="1" allow-none="1">
-            <doc xml:space="preserve" filename="../subprojects/gst-plugins-base/gst-libs/gst/pbutils/encoding-profile.c">The name of the preset to use in this @profile.</doc>
+            <doc xml:space="preserve" filename="../subprojects/gst-plugins-base/gst-libs/gst/pbutils/encoding-profile.c">The name of the element factory to use in this @profile.</doc>
             <type name="utf8" c:type="const gchar*"/>
           </parameter>
         </parameters>
@@ -3850,6 +3869,7 @@ Moreover, you can set the extra properties:
  * `|single-segment=true` (See #gst_encoding_profile_set_single_segment)
  * `|single-segment=true` (See
    #gst_encoding_video_profile_set_variableframerate)
+ * `|factory-name=element-factory-name` (See #gst_encoding_profile_set_preset_name)
 
 for example:
 

subprojects/gst-plugins-base/gst-libs/gst/pbutils/encoding-profile.c

@@ -109,6 +109,7 @@
  *  * `|single-segment=true` (See #gst_encoding_profile_set_single_segment)
  *  * `|single-segment=true` (See
  *    #gst_encoding_video_profile_set_variableframerate)
+ *  * `|factory-name=element-factory-name` (See #gst_encoding_profile_set_preset_name)
  *
  * for example:
  *
@@ -765,9 +766,26 @@ gst_encoding_profile_set_single_segment (GstEncodingProfile * profile,
  * @profile: a #GstEncodingProfile
  * @preset: (nullable): the element preset to use
  *
- * Sets the name of the #GstElement that implements the #GstPreset interface
+ * Sets the name of the preset to be used in the profile.
- * to use for the profile.
  * This is the name that has been set when saving the preset.
+ * You can list the available presets for a specific element factory
+ * using  `$ gst-inspect-1.0 element-factory-name`, for example for
+ * `x264enc`:
+ *
+ * ``` bash
+ * $ gst-inspect-1.0 x264enc
+ * ...
+ * Presets:
+ *  "Profile Baseline": Baseline Profile
+ *  "Profile High": High Profile
+ *  "Profile Main": Main Profile
+ *  "Profile YouTube": YouTube recommended settings (https://support.google.com/youtube/answer/1722171)
+ *  "Quality High": High quality
+ *  "Quality Low": Low quality
+ *  "Quality Normal": Normal quality
+ *  "Zero Latency"
+ * ```
+ }
  */
 void
 gst_encoding_profile_set_preset (GstEncodingProfile * profile,
@@ -782,9 +800,11 @@ gst_encoding_profile_set_preset (GstEncodingProfile * profile,
 /**
  * gst_encoding_profile_set_preset_name:
  * @profile: a #GstEncodingProfile
- * @preset_name: (nullable): The name of the preset to use in this @profile.
+ * @preset_name: (nullable): The name of the element factory to use in this @profile.
  *
- * Sets the name of the #GstPreset's factory to be used in the profile.
+ * Sets the name of the #GstPreset's factory to be used in the profile. This
+ * is the name of the **element factory** that implements the #GstPreset interface not
+ * the name of the preset itself (see #gst_encoding_profile_set_preset).
  */
 void
 gst_encoding_profile_set_preset_name (GstEncodingProfile * profile,
@@ -1908,6 +1928,14 @@ create_encoding_stream_profile (gchar * serialized_profile,
 
         single_segment = g_value_get_boolean (&v);
         g_value_reset (&v);
+      } else if (!g_strcmp0 (propv[0], "factory-name")) {
+        if (factory_name) {
+          g_warning ("Multiple factory names specified");
+          g_strfreev (propv);
+          goto cleanup;
+        }
+
+        factory_name = g_strdup (propv[1]);
       } else {
         g_warning ("Unsupported property: %s", propv[0]);
         g_strfreev (propv);