vaapi_encode: Maintain a pool of bitstream output buffers

vaapi_encode: Maintain a pool of bitstream output buffers

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.