Make new VDPAU easier to use by adding context to callback.

Using VDPAU correctly means checking for preemption
and possibly regenerating the context all the time.
With the current API there is no context or other
user-defined pointer and thus this in not possible
during decoding unless using some hack like global
variables.
The need to reinitialize both surfaces and even function
pointers makes handling preemption even more difficult.
This patch introduces a new render2 function that gets
both the AVCodecContext and AVFrame in addition,
in both the user can store additional opaque data.
This allows even advanced approaches like keeping a
"generation counter" for the surfaces so they can be
regenerated on the fly and efficiently.
In addition, the function has a return value that will
be passed through all the way instead of being silently
ignored as for the current render function.
Unfortunately the HWAccel API has no way of providing
API/ABI compatibility, so a currently disallowed
state (render pointer being NULL) is used to extend it.

Signed-off-by: Reimar Döffinger <Reimar.Doeffinger@gmx.de>
