[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/libavcodec/codec_list.c

@@ -0,0 +1,7 @@
+static const AVCodec * const codec_list[] = {
+    &ff_mp3float_decoder,
+    &ff_mp3_decoder,
+    &ff_wmapro_decoder,
+    &ff_wmav2_decoder,
+    &ff_xmaframes_decoder,
+    NULL };