clutter/pick-stack: Use exclusive bottom/right box borders when picking
The graphene functions used by clutter for picking assume that boxes are inclusive in both there start and end coordinates, so picking at y coordinate 32 for an actor with the height 32 placed at y coordinate 0 would still be considered a hit. This however is wrong as 32 is the first position that is not in the actor anymore.
Usually this would not be much of a problem, because motion events are rarely ever at exactly these borders and even if they are there will be another motion event soon after. But since actors in gnome-shell usually are aligned with the pixel grid and on X11 enter/leave events are generated by the X server at integer coordinates, this case is much more likely for those.
This can cause issues with Firefox which when using client side decorations, still requests MWM_DECOR_BORDER via _MOTIF_WM_HINTS to have mutter draw a border + shadow. This means that the Firefox window even when using CSD is still reparented. For such windows we receive among others XI_RawMotion and XI_Enter events, but no XI_Motion events. And the raw motion events are discarded after an enter event, because that sets has_pointer_focus to TRUE in MetaSeatX11. So when moving the cursor from the panel to a maximized Firefox window the last event clutter receives is the enter event at exactly integer coordinates. Since the panel is 32px tall and the generated enter event is at y position 32, the picking code will pick a panel actor and the focus will remain on it as long as the cursor does not leave the Firefox window.
Fix this by excluding the bottom and right border of a box when picking.