optimised versions of the get_pixels, diff_pixels, and bswap_buf

functions and support for 4 pixel high blocks in the put_pixels8_xxx and
avg_pixels8_xxx functions, courtesy of Robin Kay (komadori
at gekkou.co.uk)


git-svn-id: file:///var/local/repositories/ffmpeg/trunk@2358 9553f0bf-9b14-0410-a0b8-cfaf0461ba5b