This change introduces basic support for HEVC decoding through vdpau.
Right now, there are problems with the nvidia driver/library implementation
that mean that frames are incorrectly laid out in memory when they are
returned from the decoder, and it is normally impossible to recover the
complete decoded frame due to loss of data from alignment inconsistencies.

I obviously hope that nvidia will be fixing it in due course - I've verified
the problems exist with their example application.

As such, this support is not useful for any real world application, but I
believe that it is correct (with the caveat that the mangled frames may hide
problems) and will work properly once the nvidia problem is fixed.

Right now it appears that any file encoded by x265 or nvenc is decoded
correctly, but that's because these files don't use a bunch of HEVC
features.

Quick summary:

Features that seem to work:

1) Short Term References
2) Scaling Lists
3) Tiling

Features with known problems:

1) Long Term References

It's hard to tell what's going on here. After I read the nvidia example
app that does not set the IsLongTerm flag on LTRs, and changed my code,
a bunch of frames using LTR started to display correctly, but there
are still samples with glitches that are related to LTRs.

In terms of real world files, both x265 and nvenc only use short term
refs from this list. The divx encoder seems similar.
Signed-off-by: Philip Langdale <philipl@overt.org>