fixed THREE.PointCloud rendering

Updated the fix from https://github.com/mrdoob/three.js/pull/5628#issuecomment-63091710

Now I've basically copied the THREE.Line rendering path, but changed mode to _gl.POINTS and _this.info.render.vertices to _this.info.render.points. 

Updated example: http://potree.org/stuff/threejs/fix_indexed_pointclouds/examples/indexed_pointcloud_fixed.html