gdk/surface: Handle clicks outside client surfaces

gdk/surface: Handle clicks outside client surfaces

The owner_events=TRUE grab makes GDK on X11 see events happening
outside every client window as received on the grab window.
Additionally check that the pointer is inside the grab window
(i.e. it received GDK_CROSSING_NORMAL crossing events for the
core pointer) in order to handle clicks happening outside client
windows.

These new paths are expected to be a no-op on Wayland, and to
also work for touchscreen input on X11, due to emulated pointer
events.