libobs: Add ability for transitions to render sources directly

For specific types of transitions (stingers in this case), there is no
blending done of the two targets, so wasting GPU resources rendering
them to textures is unnecessary.

libobs/obs.h

@@ -1170,6 +1170,11 @@ EXPORT float obs_transition_get_time(obs_source_t *transition);
 EXPORT void obs_transition_video_render(obs_source_t *transition,
 		obs_transition_video_render_callback_t callback);
 
+/** Directly renders its sub-source instead of to texture.  Returns false if no
+ * longer transitioning */
+EXPORT bool obs_transition_video_render_direct(obs_source_t *transition,
+		enum obs_transition_target target);
+
 EXPORT bool obs_transition_audio_render(obs_source_t *transition,
 		uint64_t *ts_out, struct obs_source_audio_mix *audio,
 		uint32_t mixers, size_t channels, size_t sample_rate,