matroskaenc: write correct Display{Width, Height} in stereo encoding

should be the raw amount of pixels (for example 3840x1080 for full HD side by
side) and the DisplayWidth/Height in pixels should be the amount of pixels for
one plane (1920x1080 for that full HD stream)."

So, move the aspect ratio check in the mkv_write_stereo_mode() function
and always write the embl when stereo format and/or aspect ration is set.
Also add a few comments to that function.

CC: libav-stable@libav.org
Found-by: Asan Usipov <asan.usipov@gmail.com>
filmstripdec: avoid integer overflow

CC: libav-stable@libav.org
Bug-Id: CID 732246
bethsoftvid: check return value and clean memory

CC: libav-stable@libav.org
Bug-Id: CID 733777
mxfdec: reduce loop bound in mxf_read_pixel_layout()

Makes coverity less confused and code more readable.

Bug-Id: CID 732262