Only set the mouse visible/hidden if there's a window.
Only set the mouse visible/hidden if there's a window.

Fixes issue #739.

theblacklion/pyglet,oktayacikalin/pyglet,oktayacikalin/pyglet,oktayacikalin/pyglet,theblacklion/pyglet,oktayacikalin/pyglet,theblacklion/pyglet,oktayacikalin/pyglet,theblacklion/pyglet,theblacklion/pyglet