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.
