Clarify output of av_get_bits_per_pixel

Signed-off-by: Anton Khirnov <anton@khirnov.net>
avfiltergraph: simplify inserting conversion filters.

There is now no need to explicitly pass 0:0 as width/height to scale,
those are the defaults.
lavfi: fix compatibility code for old vf_scale options syntax

Currently it would incorrectly trigger on a string that contains a '='
but does not contain a ':', e.g. flags=<flags>.
jpeg2000dec: don't use deprecated PIX_FMT values
pixdesc: rename PIX_FMT_* flags to AV_PIX_FMT_FLAG_*