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

diff --git a/protocol/wayland.xml b/protocol/wayland.xml
index e3dab2e..4e59c3a 100644 (file)
--- a/protocol/wayland.xml
+++ b/protocol/wayland.xml
@@ -296,6 +296,9 @@
        The drm format codes match the macros defined in drm_fourcc.h, except
        argb8888 and xrgb8888. The formats actually supported by the compositor
        will be reported by the format event.
+
+       For all wl_shm formats and unless specified in another protocol
+       extension, pre-multiplied alpha is used for pixel values.
       </description>
       <!-- Note to protocol writers: don't update this list manually, instead
           run the automated script that keeps it in sync with drm_fourcc.h. -->