Implement proper timestamping for references

Reference frames are now identified using their timestamp:
set the timestamp when queuing the output buffer and use it to identify
the frame later on.

Signed-off-by: Paul Kocialkowski <paul.kocialkowski@bootlin.com>

src/surface.h

@@ -62,6 +62,8 @@ struct object_surface {
 	unsigned int slices_size;
 	unsigned int slices_count;
 
+	struct timeval timestamp;
+
 	union {
 		struct {
 			VAPictureParameterBufferMPEG2 picture;