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

UI/forms/OBSBasicSettings.ui

@@ -3180,6 +3180,31 @@
              <string>Stereo</string>
             </property>
            </item>
+           <item>
+            <property name="text">
+             <string>2.1</string>
+            </property>
+           </item>
+           <item>
+            <property name="text">
+             <string>4.0 Quad</string>
+            </property>
+           </item>
+           <item>
+            <property name="text">
+             <string>4.1</string>
+            </property>
+           </item>
+           <item>
+            <property name="text">
+             <string>5.1</string>
+            </property>
+           </item>
+           <item>
+            <property name="text">
+             <string>7.1</string>
+            </property>
+           </item>
           </widget>
          </item>
          <item row="2" column="0">
@@ -3309,6 +3334,19 @@
            </property>
           </widget>
          </item>
+         <item row="9" column="1">
+          <widget class="QLabel" name="audioMsg_2">
+           <property name="text">
+            <string notr="true"/>
+           </property>
+           <property name="wordWrap">
+            <bool>true</bool>
+           </property>
+           <property name="themeID" stdset="0">
+            <string notr="true">warning</string>
+           </property>
+          </widget>
+         </item>
         </layout>
        </widget>
        <widget class="QWidget" name="videoPage">
@@ -3613,8 +3651,8 @@
           <rect>
            <x>0</x>
            <y>0</y>
-           <width>98</width>
-           <height>28</height>
+           <width>818</width>
+           <height>697</height>
           </rect>
          </property>
          <layout class="QFormLayout" name="hotkeyLayout">
@@ -3659,7 +3697,7 @@
             <property name="geometry">
              <rect>
               <x>0</x>
-              <y>0</y>
+              <y>-62</y>
               <width>803</width>
               <height>761</height>
              </rect>