Don't break grab and focus state for a window when redirecting it.
Composite uses an unmap/map cycle to trigger backing pixmap allocation
and cliprect recomputation when a window is redirected or unredirected.
To avoid protocol visible side effects, map and unmap events are
disabled temporarily. However, when a window is unmapped it is also
removed from grabs and loses focus, but these state changes are not
disabled.
This change supresses the unmap side effects during the composite
unmap/map cycle and fixes a bug where a compositing window manager
would cause the screensaver to lose its grab when it unredirects the
fullscreen lock window.
