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Part-of: <!331>

Credit original authors on a per-file basis as we cannot expect people
to know all country-specific rules, or bother browsing through the git
history.

Drop obsolete GST_VAAPI_IS_xxx() helper macros since we are no longer
deriving from GObject and so those were only checking for whether the
argument was NULL or not. This is now irrelevant, and even confusing
to some extent, because we no longer have type checking.

Note: this incurs more type checking (review) but the libgstvaapi is
rather small, so this is manageable.

Add the necessary helpers in GstVaapiDisplay to determine whether subpictures
with global alpha are supported or not. Also add accessors in GstVaapiSubpicture
to address this feature.
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.

Declaring a function as const enables better optimization of calls
to the function.
Signed-off-by: Gwenole Beauchesne <gwenole.beauchesne@intel.com>