gdkwindow: Remove O(n-children) code in gdk_window_invalidate

gdkwindow: Remove O(n-children) code in gdk_window_invalidate

When we invalidate a window we need to also invalidate all child windows
that are native (non-native are automatically invalidated as we track
invalidation once per native window only). This was done in a pretty
inefficient way, recursing over the entire tree.

This makes the invalidation much faster by only looking at the native
children of the native window we're in, filtering out those that
are not a descendant of the client side window we're interested in.
Given that there are very few native subwindows this is much faster.