Handle global-alpha from GstVideoOverlayComposition API. Likewise,
the same code path could also work for premultiplied-alpha but this
was not tested.
Signed-off-by: Gwenole Beauchesne <gwenole.beauchesne@intel.com>
Add premultiplied-alpha and global-alpha feature flags, along with converters
between VA-API and gstreamer-vaapi definitions. Another round of helpers is
also necessary for GstVideoOverlayComposition API.
VA drivers may have a faster means to transfer user buffers to GPU
buffers than using memcpy(). In particular, on Intel Gen graphics, we
can use pwrite(). This provides for faster upload of bitstream and can
help higher bitrates.
vaapi_create_buffer() helper function was also updated to allow for
un-mapped buffers and pre-initialized data for buffers.
