Implement high encoder CPU usage handling

This implements the 'frame skipping' mechanism to forcibly cause frames
to be duplicated in order to reduce encoder complexity so the encoder
can catch up to the video, otherwise it will continue to be
progressively behind and will cause a desync of the video.

Typically, if a user gets this issue, they should turn down their
settings.  For the love of god do not tell them that 'frames are
skipping', just tell them that CPU usage is high, and that they should
consider turning down their settings.

libobs/media-io/video-io.h

@@ -143,6 +143,8 @@ EXPORT uint32_t video_output_width(video_t video);
 EXPORT uint32_t video_output_height(video_t video);
 EXPORT double video_output_framerate(video_t video);
 
+EXPORT uint32_t video_output_num_skipped_frames(video_t video);
+
 
 #ifdef __cplusplus
 }