avcodec, avutil: allow more control about how samples are skipped

Add CODEC_FLAG2_SKIP_MANUAL (exposed as "skip_manual"), which makes
the decoder export sample skip information via side data, instead
of applying it automatically. The format of the side data is the
same as AV_PKT_DATA_SKIP_SAMPLES, but since AVPacket and AVFrame
side data constants overlap, AV_FRAME_DATA_SKIP_SAMPLES needs to
be introduced.

This is useful for applications which want to do the timestamp
calculations manually, or which actually want to retrieve the
padding.

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