core: end-of-grab button releases must be consumed by the window

Returning FALSE here gets the button release event propagated to the
client on wayland, which is unexpected after xdg_surface.move/resize()
have been called.

https://bugzilla.gnome.org/show_bug.cgi?id=738888
wayland: Avoid MetaWindow call on non window-backed surfaces

Crossing events may also be gotten on subsurfaces.

https://bugzilla.gnome.org/show_bug.cgi?id=738890
compositor: Ensure child actors are included in picking

If the actor surface has an input mask, custom picking is implemented
for the portions affected by the mask, although the child actors (most
usually subsurfaces) are left out.

https://bugzilla.gnome.org/show_bug.cgi?id=738890