Revert "lavc/utils: Do not require dimensions for PNG."

mplayer-specifc hacks should not be in our codebase. mplayer should fix
its own code. It is not our responsibility to work around their broken
code.

This reverts commit e8e575633faf19711910cf9caf59f7db300a9ccd.

Signed-off-by: Derek Buitenhuis <derek.buitenhuis@gmail.com>
