various: Fix definition of double-buffered state

The strict "mailbox" model of wayland past is not how modern compositors
process commits, and many explanations of how double buffered state is
applied throughout wayland-protocols are no longer strictly accurate.

Instead of trying to define double-buffered state at every point of use,
just reference the evolving definition of wl_surface.commit.

This still leaves a few old definitions that weren't trivially updated.

Signed-off-by: Derek Foreman <derek.foreman@collabora.com>