Handle expose events moving or resizing windows

Handle expose events moving or resizing windows

When a window is moved or resized from a double-buffered expose handler
we can't really just copy the window region around, as the window
will be overdrawn with the double buffered region when the expose returns.

Instead we remove all regions with outstanding implicit paints from the
region to be copied and just mark this area as invalid to be redrawn
later.

This fixes bug 594880.