Fix a compile error reported by masaykh (affects cygwin/mingw gcc 4.8.1).
Fix garbled practice screen audio. May introduce regressions for older ffmpeg versions that used interleaved audio (ffmpeg version requirements should be updated). Only minimal testing done.
Workaround for mingw headers not providing std::round.
Fix resource compiling - again.

Because the SDL2 merge ate my fix...
get libavutil math class from Cmake
Merge remote-tracking branch 'refs/remotes/origin/master' into
libav09fixes

Conflicts:
	game/ffmpeg.cc