This change makes the configured GOP size be respected exactly -
previously the value could be exceeded slightly due to flaws in the
frame type selection logic.
While outwardly bizarre, this change makes the behaviour consistent
with other VAAPI encoders which sync to the encode /input/ picture in
order to wait for /output/ from the encoder. It is not harmful on
i965 (because synchronisation already happens in vaRenderPicture(),
so it has no effect there), and it allows the encoder to work on
mesa/gallium which assumes this behaviour.
This allows better checking of capabilities and will make it easier
to add more functionality later.
It also commonises some duplicated code around rate control setup
and adds more comments explaining the internals.
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.
