Replace CODEC_FLAG2_FAST with AV_CODEC_FLAG2_FAST

[Leo Izen]

FFmpeg replaced CODEC_FLAG2_FAST with AV_CODEC_FLAG2_FAST two years ago and left CODEC_FLAG2_FAST as a deprecated alias. This deprecated alias has finally been removed two years later, so it needs to be switched over to AV_CODEC_FLAG2_FAST or Xpra won't compile with a recent version of FFmpeg. This doesn't break compatibility with stable systems, since AV_CODEC_FLAG2_FAST has existed for more than two years. This change was already made in the Xpra codebase for CODEC_ID_H264 -> AV_CODEC_ID_H264 and similar changes, but not for AV_CODEC_FLAG2_FAST.

[Antoine Martin]

Already been done in r18088, r18087, r18086.

Make sure the version you use is up to date.