Game should not override cursor sprite when over window frame.

[Jayschwa]

The game normally uses a custom cursor sprite when the cursor is within the gameplay area. However, the custom sprite is still displayed when the cursor moves outside of the gameplay area and over the window's frame. The cursor should go back to whatever the desktop environment wants at this point. It is slightly disconcerting to still see a game pointer when one expects to see a resize sprite.

[ben]

(In [8789]) Handle SDL_ACTIVEEVENT which notifies us of mouse focus change. If mouse focus is lost don't render cursor, reverting back to OS style. When focus is regained, render cursor again. Fixes #626