gdk: Don't ever do copies from the window

gdk: Don't ever do copies from the window

This basically neuters gdk_window_move_region, gdk_window_scroll
and gdk_window_move_resize, in that they now never copy any bits but
just invalidate the source and destination regions. This is a performance
loss, but the hope is that the simplifications it later allows will let
us recover this performance loss (which mainly affects scrolling).