Simplify gst_vaapiencode_push_frame(), while also removing the call
to gst_video_encoder_negotiate() since this is implicit in _finish()
if caps changed. Also fixed memory leaks that occured on error.
Constify pointers wherever possible. Drop unused variables, and use
consistent variable names. Fix gst_vaapiencode_h264_allocate_buffer()
to correctly report errors, especially when in-place conversion from
bytestream to avcC format failed.
Move "rate-control" mode and "bitrate" properties to the GstVaapiEncode
base class. The actual range of supported rate control modes is currently
implemented as a plug-in element hook. This ought to be determined from
the GstVaapiEncoder object instead, i.e. from libgstvaapi.
Add a GST_VAAPIENCODE_CAST() helper to avoid run-time checks against
the GObject type system. We are guaranteed to only deal with the same
plug-in element object.
Allow vaapiencode plug-in elements to encode from raw YUV buffers.
The most efficient way to do so is to let the vaapiencode elements
allocate a buffer pool, and subsequently buffers from it. This means
that upstream elements are expected to honour downstream pools.
If upstream elements insist on providing their own allocated buffers
to the vaapiencode elements, then it possibly would be more efficient
to insert a vaapipostproc element before the vaapiencode element.
This is because vaapipostproc currently has better support than other
elements for "foreign" raw YUV buffers.
