matroskaenc: simplify mkv_check_tag()

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
Fixing a stride issue in VP8 Alpha encode

When encoding alpha channel in libvpx, the stride isn't set
properly for the alpha encoder. Fixing it.

Signed-off-by: Vignesh Venkatasubramanian <vigneshv@google.com>
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>