[mythtv] Ticket #12531: vdpau segfaults when video dimensions not multiple of 16

[Jean-Yves Avenard]

> Sorry for replying late but I was waiting for the code.mythtv.org ticket to
> update with your comments (which it hasn't done yet). So, yes, I created a
> bug report on nvidia devtalk.
>
> https://devtalk.nvidia.com/default/topic/909797/linux/vdpau-segfaults-when-videos-dimensions-are-not-multiple-of-16/

Actually, upon looking at the code, I believe how the plans offset and
stride are calculated is wrong: causing an out of bound access.. Plus
I'm unsure why we would need to clear the nvidia surface before
uplifting another surface.