The "constrained" property for a mouse mode now actually constrains

The "constrained" property for a mouse mode now actually constrains
the mouse rather than wrapping it.  Wrapping around to the other side
of the screen has very bad consequences when using the mouse for
flying or viewing -- it can result in sudden jumps in the controls or
the viewpoint when the mouse jumps to another side of the screen.

Right now, the mouse is constrained to stay between 25% and 75% of the
screen on both the X and Y axis -- whenever it hits an edge, it jumps
back to the centre of the screen again (which causes no control or
view jump).