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>
hevc_mvs: set candidate availabilities

They might be left uninitialized otherwise since 3ad04608.

Fixes ticket #3840.

Found-by: Carl Eugen Hoyos <ce@hoyos.ws>
Reported-by: Piotr Bandurski <ami_stuff@o2.pl>

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>