Fix image size restrictions set by r35242.

Luma blocks are 16x16, but chroma blocks are (two interweaved) 8x16.
Since chroma is at quarter resolution it means that the image
width must be multiple of 16, while
height must be multiple of 32.


git-svn-id: svn://svn.mplayerhq.hu/mplayer/trunk@36979 b3059339-0415-0410-9bf9-f77b7e298cf2