gdk: Fix invalidation w/ pixel cache when changing child window geometry.

gdk: Fix invalidation w/ pixel cache when changing child window geometry.

When moving/scrolling a child window we can't use the current clip
region to limit what is invalidated, because there may be a pixel
cache that listens for changes outside the clip region. Instead
invalidate the entire area and rely on the invalidation code to limit
the repaint to the actually visible area.