UI: Add VOD track support in advanced output

Adds a VOD track option (specific to Twitch) that allows a user to
specify which audio track to use for their Twitch VODs, which uses a
separate encoder to encode the track. This allows users the ability to
choose what audio goes on their VOD, separately from the live stream.

UI/window-basic-main.cpp

@@ -1301,6 +1301,7 @@ bool OBSBasic::InitBasicConfigDefaults()
 				true);
 	config_set_default_bool(basicConfig, "AdvOut", "UseRescale", false);
 	config_set_default_uint(basicConfig, "AdvOut", "TrackIndex", 1);
+	config_set_default_uint(basicConfig, "AdvOut", "VodTrackIndex", 2);
 	config_set_default_string(basicConfig, "AdvOut", "Encoder", "obs_x264");
 
 	config_set_default_string(basicConfig, "AdvOut", "RecType", "Standard");