Prevent RHI graphics resource leaks once the 5.15->dev merge is in place.

Do a 5.15->dev merge (conflicts, port changed behavior, test, etc.). This will pull in a number of changes that allow better cleanup on application exit.

Once this is in place, we need to follow suit in dev-only places (for example everything QSSGRhiContext related) to prevent QSSGRefs with refcount never reaching zero. This should result in massively reducing the graphics resource leak warnings when closing applications.

Once that is done, investigate the remaining leaks: why some QRhiBuffer/Texture instances are not destroyed properly.