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/obs-libfdk/obs-libfdk.c

@@ -131,6 +131,16 @@ static void *libfdk_create(obs_data_t *settings, obs_encoder_t *encoder)
 	case 6:
 		mode = MODE_1_2_2_1;
 		break;
+
+        /* lib_fdk-aac > 1.3 required for 7.1 surround;
+         * uncomment if available on linux build
+         */
+#ifndef __linux__
+	case 8:
+		mode = MODE_7_1_REAR_SURROUND;
+		break;
+#endif
+
 	default:
 		blog(LOG_ERROR, "Invalid channel count");
 		goto fail;