mpeg2dec: Fix handling of stride

mpeg2dec: Fix handling of stride

A pipeline like:

gst-launch-1.0 filesrc location=file.ts ! tsdemux ! mpegvideoparse ! mpeg2dec  ! vaapisink

would look bad when file.ts contains 704x576 video, because vaapisink would
give you buffers of stride 768, but libmpeg2 was not told about this and
used a stride of 704.

Tell libmpeg2 about the stride from  downstream; in the process, teach it to
reject buffer pools that don't meet libmpeg2's chroma stride requirements

Signed-off-by: Simon Farnsworth <simon.farnsworth@onelan.co.uk>