Fix gestures test

This tests was testing gestures by faking an event in the middle of a window that has a hbox with an expanding image in it. For me (and I guess this depends on all sorts of issues like whether CSD is enabled, font sizes, etc) the hbox ended up centered horizontally but not vertically (probably because of csd at the top), so no events ever hit the inner widgets.

This is fixed by emitting the events at allocation.x/y of the hbox, which should contain both the hbox and the image (as it expands).