libobs: Add obs_video_active() function

(This commit also modifies UI)

Adds a universal function for determining whether video output is
currently active, rather than having to use video_output_active() on the
value returned by obs_get_video().

libobs/obs.h

@@ -535,6 +535,9 @@ EXPORT audio_t *obs_get_audio(void);
 /** Gets the main video output handler for this OBS context */
 EXPORT video_t *obs_get_video(void);
 
+/** Returns true if video is active, false otherwise */
+EXPORT bool obs_video_active(void);
+
 /** Sets the primary output source for a channel. */
 EXPORT void obs_set_output_source(uint32_t channel, obs_source_t *source);