v4l2: Workaround driver not setting field correctly

As it's very common, handle driver not setting field in buffers
by using the field value from the format. This workaround a long time
bug in UVC driver. For even buggier driver, we simply assume
progressive as before. We also only warn once, to avoid spamming.

sys/v4l2/gstv4l2bufferpool.c

@@ -1164,6 +1164,34 @@ gst_v4l2_buffer_pool_dqbuf (GstV4l2BufferPool * pool, GstBuffer ** buffer)
   }
 #endif
 
+  /* Check for driver bug in reporting feild */
+  if (group->buffer.field == V4L2_FIELD_ANY) {
+    /* Only warn once to avoid the spamming */
+#ifndef GST_DISABLE_GST_DEBUG
+    if (!pool->has_warned_on_buggy_field) {
+      pool->has_warned_on_buggy_field = TRUE;
+      GST_WARNING_OBJECT (pool,
+          "Driver should never set v4l2_buffer.field to ANY");
+    }
+#endif
+
+    /* Use the value from the format (works for UVC bug) */
+    group->buffer.field = obj->format.fmt.pix.field;
+
+    /* If driver also has buggy S_FMT, assume progressive */
+    if (group->buffer.field == V4L2_FIELD_ANY) {
+#ifndef GST_DISABLE_GST_DEBUG
+      if (!pool->has_warned_on_buggy_field) {
+        pool->has_warned_on_buggy_field = TRUE;
+        GST_WARNING_OBJECT (pool,
+            "Driver should never set v4l2_format.pix.field to ANY");
+      }
+#endif
+
+      group->buffer.field = V4L2_FIELD_NONE;
+    }
+  }
+
   /* set top/bottom field first if v4l2_buffer has the information */
   switch (group->buffer.field) {
     case V4L2_FIELD_NONE:

sys/v4l2/gstv4l2bufferpool.h

@@ -89,6 +89,9 @@ struct _GstV4l2BufferPool
 
   /* signal handlers */
   gulong group_released_handler;
+
+  /* Control to warn only once on buggy feild driver bug */
+  gboolean has_warned_on_buggy_field;
 };
 
 struct _GstV4l2BufferPoolClass