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.

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