It should be more efficient than u_blitter since it skips
vertex shader stage. Also it's a prerequisite for supporting
MSAA since u_blitter can't do MSAA resolve for us.
Reviewed-by: Erico Nunes <nunes.erico@gmail.com>
Signed-off-by: Vasily Khoruzhick <anarsoul@gmail.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/16686>