Update XvMC implementation to the changes in FFmpeg.

Use the common AV_PIX_FMT_XVMC.
Remove usage of avctx->xvmc_acceleration and CODEC_CAP_HWACCEL.
Return error instead of exiting.

git-svn-id: svn://svn.mplayerhq.hu/mplayer/trunk@37151 b3059339-0415-0410-9bf9-f77b7e298cf2
Disable xvid if xvid_plugin_ssim_t is not present.
It have been introduced 8 years ago without version bump.
Required by FFmpeg libxvid.


git-svn-id: svn://svn.mplayerhq.hu/mplayer/trunk@37152 b3059339-0415-0410-9bf9-f77b7e298cf2