avfilter: add mergeplanes filter

Signed-off-by: Paul B Mahol <onemda@gmail.com>
correct the AVOption documentation for AV_EF_CAREFUL

The flag was and is intended to be a sane default and thus does not enable
time consuming checks. This also matches how the flag is used in decoders and
demuxers.

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
avcodec/avcodec.h: Add documentation for the AV_EF_* defifines

The documentation is copied from the AVOption texts

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>