plugins: set negotiation caps in src allocator

When the allocator is created, it stores the allocation caps. But sometimes
the "allocation caps" may be different from the "negotiation caps".

In this case, the allocator should store the negotiation caps since they
are the ones used for frame mapping with GstVideoMeta.

When vaapispostproc is used, this is not a problem since the element is assume
to resize. But when using a vaapi decoder only, with a software renderer, it
fails in this case.

https://bugzilla.gnome.org/show_bug.cgi?id=773323

gst/vaapi/gstvaapipluginbase.c

@@ -545,10 +545,27 @@ static gboolean
 ensure_srcpad_allocator (GstVaapiPluginBase * plugin, GstVideoInfo * vinfo,
     GstCaps * caps)
 {
+  gboolean different_caps;
+  GstVideoInfo vi;
   GstVaapiImageUsageFlags usage_flag =
       GST_VAAPI_IMAGE_USAGE_FLAG_NATIVE_FORMATS;
 
-  if (!reset_allocator (plugin->srcpad_allocator, vinfo))
+  /* the received caps are the "allocation caps" which may be
+   * different from the "negotiation caps". In this case, we should
+   * indicate the allocator to store the negotiation caps since they
+   * are the one should be used for frame mapping with GstVideoMeta */
+  different_caps = GST_IS_VIDEO_DECODER (plugin) && plugin->srcpad_caps &&
+      !gst_caps_is_strictly_equal (plugin->srcpad_caps, caps);
+
+  if (different_caps) {
+    vi = plugin->srcpad_info;
+    /* let's keep the size of the allocation info */
+    GST_VIDEO_INFO_SIZE (&vi) = GST_VIDEO_INFO_SIZE (vinfo);
+  } else {
+    vi = *vinfo;
+  }
+
+  if (!reset_allocator (plugin->srcpad_allocator, &vi))
     return TRUE;
 
   /* enable direct rendering if downstream requests raw video */
@@ -561,6 +578,10 @@ ensure_srcpad_allocator (GstVaapiPluginBase * plugin, GstVideoInfo * vinfo,
       gst_vaapi_video_allocator_new (plugin->display, vinfo, 0, usage_flag);
   if (!plugin->srcpad_allocator)
     goto error_create_allocator;
+
+  if (different_caps)
+    gst_allocator_set_vaapi_video_info (plugin->srcpad_allocator, &vi, 0);
+
   return TRUE;
 
   /* ERRORS */