mpeg2: disable video cropping as picture_display_extension() is missing.

mpeg2: disable video cropping as picture_display_extension() is missing.

Disable video cropping in MPEG-2 codec because it is partially implemented
and actually because nobody implements it that way, and the standard spec
does not specify the display process either anyway.

Most notably, there are two possible use cases for sequence_display_extension()
horizontal_display_size & vertical_display_size: (i) guesstimating the
pixel-aspect-ratio, or (ii) implement some kind of span & scan process
in conjunction with picture_display_extension() information.

https://bugzilla.gnome.org/show_bug.cgi?id=704848