v4l2src: Add source-change / resolution-change support

This branch adds support for driver driven source changes along with DV_TIMING locking support. Effectively, drivers can emit SOURCE_CHANGE event, with the RESOLUTION_CHANGE flag in order to signal that something got changes at the input signal. In that situation, we need to reset (streamoff/on) as we may not get more data otherwise. While doing this, we now query for detected DV_TIMING (aka modes) and lock/set these. We also default to the link detected width/height/framerate.

As a bonus, v4l2src will now mimic decklinksrc when the signal is lost and recovered. A WARNING with RESSOURCE/READ will be emited when the signal is lost, and an INFO message again with RESSOURCE/READ for when it's recovered.

This has been tested with vivid test driver and an work in progress HDMI receiver driver.

sys/v4l2/gstv4l2bufferpool.h

@@ -41,6 +41,10 @@ G_BEGIN_DECLS
 #define GST_V4L2_BUFFER_POOL(obj)      (G_TYPE_CHECK_INSTANCE_CAST ((obj), GST_TYPE_V4L2_BUFFER_POOL, GstV4l2BufferPool))
 #define GST_V4L2_BUFFER_POOL_CAST(obj) ((GstV4l2BufferPool*)(obj))
 
+/* Returns true if the pool is streaming. Must be called with stream lock
+ * held. */
+#define GST_V4L2_BUFFER_POOL_IS_STREAMING(obj) (GST_V4L2_BUFFER_POOL (obj)->streaming)
+
 /* This flow return is used to indicate that the last buffer has been dequeued
  * during draining. This should normally only occur for mem-2-mem devices. */
 #define GST_V4L2_FLOW_LAST_BUFFER GST_FLOW_CUSTOM_SUCCESS