libobs: Allow disabling of async video texture

If an async video source stops video for whatever reason, it would get
stuck on the last frame that was played.  This was particularly awkward
when I wanted to give the user the ability to deactivate a source such
as a webcam because it would get stuck on the last frame.

libobs/obs.h

@@ -802,7 +802,7 @@ EXPORT void obs_source_draw_set_color_matrix(
 EXPORT void obs_source_draw(gs_texture_t *image, int x, int y,
 		uint32_t cx, uint32_t cy, bool flip);
 
-/** Outputs asynchronous video data */
+/** Outputs asynchronous video data.  Set to NULL to deactivate the texture */
 EXPORT void obs_source_output_video(obs_source_t *source,
 		const struct obs_source_frame *frame);