Move from only supporting the default of pre-multiplied alpha to supporting user specified blend mode using the standardised property. Signed-off-by: Dave Stevenson <dave.stevenson@raspberrypi.com> Link: https://lore.kernel.org/r/20220613144800.326124-7-maxime@cerno.tech Signed-off-by: Maxime Ripard <maxime@cerno.tech>