Doomsday Engine

Canvas: Mouse tracking

Tracks the global mouse position and keeps the cursor centered on
the desktop (cursor hidden).

The new default windowed mode behavior is friendly grabbing:
the mouse is not grabbed by default, only when the mouse is clicked
inside the window. Switching focus out of the window releases the
mouse, after which a click is again needed to re-engage the grabbing.

This allows easy manipulation of the window without unwanted
grabbing.