We previously assumed that the first preferred sample format is planar, but that may not be true for all codecs. Instead we should find a supported sample format that is planar.
While YUV420P is widely used, not all encoders accept it (e.g. Intel QSV only accepts NV12). We should use the codec's preferred pixel format instead as we need to rescale the frame anyway.
This uses the mailbox model to move pixel downloading to its own thread, eliminating Nvidia's warnings and (possibly) making use of GPU copy engine.
To achieve this, we created a new mailbox type that is different from the presentation mailbox in that it never discards a rendered frame.
Also, I tweaked the projection matrix thing so that it can just draw the frame upside down instead of having the CPU flip it.
