Fix mouse control, add mouse rectangle, enable mouse by default

Fix mouse control, add mouse rectangle, enable mouse by default

Fix problems with mouse control (lock the mouse if
we are using the keyboard). This makes the mouse
control work nicely together with the keyboard control,
so we enable mouse control by default now.

Draw a mouse rectangle on the screen so the user knows
what's going on when using mouse control. This should
make playing Tennix a bit easier with the mouse, because
the racket is always sloppy (you can move the mouse faster
than you can move the racket.