Previously we would allocate a new one for every frame. This instead
maintains an AVBufferPool of them to use as-needed.
Also makes the maximum size of an output buffer adapt to the frame
size - the fixed upper bound was a bit too easy to hit when encoding
large pictures at high quality.
This prevents attempts to use unsupported modes, such as low-power
H.264 mode on non-Skylake targets. Also fixes a crash on invalid
configuration, when trying to destroy an invalid VA config/context.
