Add new FFmpeg AMVF tag for AMV video.

git-svn-id: svn://svn.mplayerhq.hu/mplayer/trunk@37267 b3059339-0415-0410-9bf9-f77b7e298cf2
Fix incorrect use of pal2gray_alpha for av_sub.

It would clear previously draw subtitle rectangles
at the same vertical position, and it could potentially
write outside the destination.

git-svn-id: svn://svn.mplayerhq.hu/mplayer/trunk@37268 b3059339-0415-0410-9bf9-f77b7e298cf2