Turn the system mouse cursor back on before VideoQuit().

This is good policy, so it doesn't have a chance to leave it hidden on targets
 that wouldn't necessarily reset it by default, but it also fixes a crash if
 you try to use a message box after SDL_Quit() is called.

Fixes Bugzilla #1969.
Mac: Fix SDL_WarpMouseInWindow in fullscreen.

If you switched from a window to fullscreen, your SDL_WarpMouseInWindow
calls would be offset by the x and y coordinates of the original window.
Mac: CGMakePoint -> CGPointMake in previous change.
Mac: Bring back FS windows when appropriate

This automatically restores FS windows when the application is made
active (Cmd-Tab, you click the Dock icon, or you launch the .app again).