mpeg4videodec: split initializing static tables into a separate function

Signed-off-by: Anton Khirnov <anton@khirnov.net>
mpeg4video_parser: init mpeg4 static tables.

They are used when decoding the frame header.
Found-by: Mateusz "j00ru" Jurczyk and Gynvael Coldwind
CC:libav-stable@libav.org

Signed-off-by: Anton Khirnov <anton@khirnov.net>
h264/mpegvideo: do not provide pixel formats for hwaccels that are not compiled in