Fix object mask cursor indicator leaking under overlay windows

[andriiryzhkov]

If you're in the object mask creation mode and you open preferences (or any other window) over the darkroom without leaving the mask tool, the mask's "+" cursor indicator keeps tracking the cursor underneath the overlay – you can see it moving on the darkroom behind the prefs window as you move the mouse over the prefs widgets.

Root cause: the mask tool's post-expose handler calls gdk_window_get_device_position() every redraw to figure out where to draw the indicator. That function returns the pointer's coordinates relative to the darkroom window regardless of which window the cursor is actually over, so as long as the coordinates land inside the canvas bounds, the indicator gets drawn there – even when the cursor is visually somewhere else.

Fix: also ask gdk_device_get_window_at_position() which window the pointer is actually over, and skip the indicator if it's not the darkroom canvas.

[TurboGit]

Thanks!