fix bug in premultipledAlpha = true with MultiplyBlending.
fix bug in premultipledAlpha = true with MultiplyBlending.

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