[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/premake5.lua

@@ -0,0 +1,42 @@
+group("third_party")
+
+--
+-- Call this function in subproject scope to set the needed include
+-- dirs and defines.
+--
+function ffmpeg_common()
+  defines({
+    "HAVE_AV_CONFIG_H",
+    "_USE_MATH_DEFINES", -- For M_PI/etc
+  })
+
+  -- Paths are relative to the calling script (sub-library premake files),
+  -- which are one level deeper (e.g. libavcodec/premake5.lua).
+  includedirs({
+    "..",           -- ffmpeg-xenia/ (config.h, avconfig.h, ffversion.h)
+    "../../FFmpeg", -- FFmpeg source headers
+  })
+  filter({"platforms:Windows", "configurations:Debug or configurations:Checked"})
+    optimize("Size") -- dead code elimination is mandatory
+    removebuildoptions({
+      "/RTCsu",      -- '/O1' and '/RTCs' command-line options are incompatible
+    })
+  filter({"platforms:Windows", "configurations:Release"})
+    linktimeoptimization("Off")
+  filter("platforms:Windows")
+    includedirs({
+      "../../FFmpeg/compat/atomics/win32",
+    })
+    links({
+      "bcrypt",
+    })
+  filter("platforms:Linux")
+    includedirs({
+      "../../FFmpeg/compat/atomics/gcc",
+    })
+  filter({})
+end
+
+include("libavcodec/premake5.lua")
+include("libavformat/premake5.lua")
+include("libavutil/premake5.lua")