[3PP] Switch FFmpeg to latest release with port of xmaframes codec

And pull out all the xenia specific build junk into
third_party/ffmpeg-xenia rather than keeping it checked in the
submodule

third_party/ffmpeg-xenia/libavformat/muxer_list.c

@@ -0,0 +1,2 @@
+static const AVOutputFormat * const muxer_list[] = {
+    NULL };