obs-ffmpeg: Add HLS output

Add and register an obs_output_info struct called
ffmpeg_hls_muxer. It uses ffmpeg's HLS muxer to stream output.

Add threading and buffer to reduce skipped frames.
Also add frame drop logic when there are too many packets in the
buffer for congestion control.

plugins/obs-ffmpeg/CMakeLists.txt

@@ -30,6 +30,7 @@ set(obs-ffmpeg_SOURCES
 	obs-ffmpeg-nvenc.c
 	obs-ffmpeg-output.c
 	obs-ffmpeg-mux.c
+	obs-ffmpeg-hls-mux.c
 	obs-ffmpeg-source.c)
 
 if(UNIX AND NOT APPLE)