Up until now, the list of pixfmts is reallocated every time an entry
is added to it; there are currently 196 pixel formats, so this matters:
It causes 5541704 calls to av_realloc_array() in a typical FATE run,
which is the majority for said function (8095768 calls) and even
a large chunk of the calls to av_realloc() itself (12589508 calls).
Fix this by using ff_formats_pixdesc_filter() instead.
Reviewed-by: Nicolas George <george@nsup.org>
Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@outlook.com>