Fix support for global-alpha subpictures. The previous changes brought
the ability to check for GstVideoOverlayRectangle changes by comparing
the underlying pixel buffer pointers. If sequence number and pixel data
did not change, then this is an indication that only the global-alpha
value changed. Now, try to update the underlying VA subpicture global-alpha
value.

Signed-off-by: Gwenole Beauchesne <gwenole.beauchesne@intel.com>