It makes it perfectly clear that we should not be modifying them.
Should help highlight issues like the one fixed with previous commit.
Fixes: cef12efc15 ("glamor: Implement GetSupportedModifiers")
Cc: Louis-Francis Ratté-Boulianne <lfrb@collabora.com>
Cc: Daniel Stone <daniels@collabora.com>
Reviewed-by: Adam Jackson <ajax@redhat.com>
Signed-off-by: Emil Velikov <emil.velikov@collabora.com>