avformat/microdvd: skip BOM properly

The BOM is already skipped in the probe function, but not the header
read function. This could cause the header to be misparsed, such as
not interpreting the FPS line.
avformat/microdvd: export the declared frame rate

MicroDVD files are normally frame-based (i.e. there are no absolute
timestamps), but can have an optional frame rate header. If this
header is missing, the timestamps depend on the frame rate of the
video they were created for.

The demuxer will use a fallback frame rate if it's missing from the
header. Currently, applications using libavformat can't know whether
the time base is based on the fallback value, or if a frame rate
header was present.

This commit introduces a subfps AVOption for MicroDVD, and the
demuxer sets it if and only if a frame rate header was present.

Signed-off-by: Clément Bœsch <u@pkh.me>
avcodec/h263dec: Workaround H263 end padding bug

Fixes: H263PaddingBugFixVID_6647.MOV

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