libobs: Add surround sound audio support

(This commit also modifies the following modules: UI,
deps/media-playback, coreaudio-encoder, decklink, linux-alsa,
linux-pulseaudio, mac-capture, obs-ffmpeg, obs-filters, obs-libfdk,
obs-outputs, win-dshow, and win-wasapi)

Adds surround sound audio support to the core, core plugins, and user
interface.

Compatible streaming services: Twitch, FB 360 live
Compatible protocols: rtmp / mpeg-ts tcp udp
Compatible file formats: mkv mp4 ts  (others untested)
Compatible codecs: ffmpeg aac, fdk_aac, CoreAudio aac,
		   opus, vorbis, pcm (others untested).
Tested streaming servers: wowza, nginx
	 HLS, mpeg-dash : surround passthrough
Html5 players tested with live surround:
	 videojs, mediaelement, viblast (hls+dash), hls.js
Decklink: on win32, swap channels order for 5.1 7.1
         (due to different channel mapping on wav, mpeg, ffmpeg)
Audio filters: surround working.
Monitoring: surround working (win macOs linux (pulse-audio)).
VST:	 stereo plugins keep in general only the first two channels.
	 surround plugins should work (e.g. mcfx does).
OS: win, macOs, linux (alsa, pulse-audio).
Misc: larger audio bitrates unlocked to accommodate more channels
NB: mf-aac only supports mono and stereo + 5.1 on win 10
         (not implemented due to lack of usefulness)

Closes jp9000/obs-studio#968

plugins/coreaudio-encoder/encoder.cpp

@@ -601,6 +601,29 @@ static void *aac_create(obs_data_t *settings, obs_encoder_t *encoder)
 			kAudioConverterCurrentOutputStreamDescription,
 			&size, &out));
 
+	/*
+	 * Fix channel map differences between CoreAudio AAC, FFmpeg, Wav
+	 * New channel mappings below assume 4.1, 5.1, 7.1 resp.
+	 */
+	if (ca->channels == 5) {
+		SInt32 channelMap5[5] = {2, 0, 1, 3, 4};
+		AudioConverterSetProperty(ca->converter,
+				kAudioConverterChannelMap,
+				sizeof(channelMap5), channelMap5);
+
+	} else if (ca->channels == 6) {
+		SInt32 channelMap6[6] = {2, 0, 1, 4, 5, 3};
+		AudioConverterSetProperty(ca->converter,
+				kAudioConverterChannelMap,
+				sizeof(channelMap6), channelMap6);
+
+	} else if (ca->channels == 8) {
+		SInt32 channelMap8[8] = {2, 0, 1, 6, 7, 4, 5, 3};
+		AudioConverterSetProperty(ca->converter,
+				kAudioConverterChannelMap,
+				sizeof(channelMap8), channelMap8);
+	}
+
 	ca->in_frame_size     = in.mBytesPerFrame;
 	ca->in_packets        = out.mFramesPerPacket / in.mFramesPerPacket;
 	ca->in_bytes_required = ca->in_packets * ca->in_frame_size;