doc/filters: apply misc fixes to drawbox documentation
lavfi/decimate: use named options
doc/filters: rework introductory description for decimate

In particular, dedicate first paragraph to deliver a short description of
the filter.
lavfi/overlay: implement shortest option

Force termination when the overlay stream ends. Simplify scripting logic,
for example when an infinite source is used to generate a background for
a composite video.
lavfi/overlay: apply minor consistency fixes
lavfi/overlay: add format option

In particular, fix misbehavior in case main and overlay input formats
mismatch (e.g. YUV420 and YUV444).
lavfi/subtitles: fix recently introduced warning about discarded const qualifier.
h264: call alloc_scratch_buffers() only once linesize is known

Fixes out of array accesses

Found-by: Mateusz "j00ru" Jurczyk and Gynvael Coldwind
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
h264: Fix handling of changing reference counts between slices

Fixes out of array accesses

Found-by: Mateusz "j00ru" Jurczyk and Gynvael Coldwind
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>