fix MaskPass's prototype.
fix MaskPass's prototype.

bhouston/three.js,bhouston/three.js,Jozain/three.js,Jozain/three.js,bhouston/three.js,Jozain/three.js,Jozain/three.js,bhouston/three.js,Jozain/three.js,bhouston/three.js,bhouston/three.js,Jozain/three.js