Fix libdecor window decorations breaking when started in gnome-shell overview mode

Currently, calculating the paint volume when in gnome-shell's overview mode will not take into account subsurfaces that extends outside the main window surface as all the subsurfaces are unmapped when in such state, breaking applications that relies on window-extending subsurfaces when started while gnome-shell is still in overview mode, such as any application using libdecor.

Fix this by installing our own ClutterActor::get_paint_volume method that works almost exactly the same as the original except it unconditionally includes all surfaces in the calculation without checking whether the surface is mapped or not.

Fixes #2350 (closed), #2114 (closed)

cc @rmader