These days exposure happens only on the native windows (generally the
toplevel window) and is propagated down recursively. The expose event
is only useful for backwards compat, and in fact, for double buffered
widgets we totally ignore the event (and non-double buffering breaks
on wayland).

So, by not setting the mask we avoid emitting these events and then
later ignoring them.

We still keep it on eventbox, fixed and layout as these are used
in weird ways that want backwards compat.