wayland: Remove an extra reset

We already reset the double-buffered state when we commit it, so this
is just superfluous.
xwayland: Make sure to clear an existing surface if we have one

This fixes an assert fail when redecorating an X11 client.
display: Rework event spewing

Make it a compile-time flag rather than a run-time flag, because
practically any time you're going to be debugging event spewing,
you're going to have to recompile anyway. Remove the WITH_VERBOSE_MODE
checks, too.