outputs RGBA. This patch supports 6 output formats and prints an error message
if it is asked to provide an output format it is not capable of.
patch by Alan Curry, pacman_at_world_dot_std_dot_com


git-svn-id: file:///var/local/repositories/mplayer/trunk/postproc@17561 b3059339-0415-0410-9bf9-f77b7e298cf2