avfilter/vf_mp:  preserve pixel format when possible

Fixes Ticket2577

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
Fix muxing QDM2 mono into caf.
Set block_align when reading QDM2 in mov.

Fixes ticket #2718.
Avoid a null pointer dereference in avcodec_decode_video2().

This could happen if oom occured while probing a file.
Reported in ticket #1900.