libobs: Make filter_video callback param non-const

The frame is definitely meant to be modifiable if needed, so it
shouldn't be const.  I originally meant for these frames to be
duplicated, but with the source video cache that's both unnecessary and
would reduce performance.

libobs/obs-source.h

@@ -250,7 +250,7 @@ struct obs_source_info {
 	 *                be drawn later if time is needed for processing
 	 */
 	struct obs_source_frame *(*filter_video)(void *data,
-			const struct obs_source_frame *frame);
+			struct obs_source_frame *frame);
 
 	/**
 	 * Called to filter raw audio data.