doc/libavutil: add things from avutil.txt

Many minor fixes from: Stefano Sabatini <stefasab@gmail.com>.

Signed-off-by: Timothy Gu <timothygu99@gmail.com>
Signed-off-by: Stefano Sabatini <stefasab@gmail.com>
doc: remove avutil.txt

No useful contents. Also hard to keep up-to-date.

Signed-off-by: Timothy Gu <timothygu99@gmail.com>
Signed-off-by: Stefano Sabatini <stefasab@gmail.com>
h264: Do not treat the initial frame special in handling of frame gaps

The not handling of frame gaps has lead to the lack of a dummy reference
frame, which has lead to the failure of decode_slice_header() which has
lead to one SEI recovery message being skiped which had introduced a
slightly suboptimal recovery point for at least 1 h264 file compared to
JM.

Found-by: Carl & BugMaster
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>