gdkwindow: Allow gdk_window_set_opacity on non-native children

gdkwindow: Allow gdk_window_set_opacity on non-native children

We now store the current opacity for all windows. For native windows
we just call into the native implementation whenever the opacity changes.
However, for non-native windows we implement opacity by pushing a
second implicit paint that "stacks" on the existing one, acting as
an opacity group while rendering the window and its children.

This works well in general, although any native child windows will of
course not be opaque. However, there is no way to implement
implicit paint flushing (i.e. draw the currently drawn double buffer
to the window in order to allow direct drawing to the window).
We can't flush in the stacked implicit paint case because there
is no way to get the right drawing behaviour when drawing directly
to the window. We *must* draw to the opacity group to get the right
behaviour.

We currently flush if:
* A widget disables double buffering
* You call move/resize/scroll a window and it has non-native children
  during the expose handler

In case this happens we warn and flush the outermost group, so there may
be drawing errors.

https://bugzilla.gnome.org/show_bug.cgi?id=687842