protocol: wl_shm uses pre-multiplied alpha

Add a note about pre-multiplied alpha for all wl_shm formats.
Include an escape hatch in the spec to allow other protocol
extensions to override this.

Signed-off-by: Simon Ser <contact@emersion.fr>
Closes: https://gitlab.freedesktop.org/wayland/wayland/-/issues/61
1 file changed