Add MPEG 2 codec

This patch introduces the support of MPEG2 video decoding. It basically
just copies data from VA's frame/slice headers data structures to v4l's
MPEG2 frame header extended controls data structures.

src/context.h

@@ -50,6 +50,8 @@ struct object_context {
 	int flags;
 	VASurfaceID *render_targets;
 	uint32_t num_rendered_surfaces;
+
+	struct v4l2_ctrl_mpeg2_frame_hdr mpeg2_frame_hdr;
 };
 
 typedef struct object_context *object_context_p;