Added billboard particles (semi-opaque depth sorted).

Work in progress, caveats:

- sort happens just inside one ParticleSystems, so everything gets messed up when more ParticleSystems are in the same screen space
- not yet solved proper order of operations, if you change object transform, you need to update ParticleSystem object matrix manually before rendering (so that sort works, otherwise it will use transform from previous frame)

Also fixed transparency in fog and did small optimizations in WebGLRenderer.

src/objects/ParticleSystem.js

@@ -8,6 +8,8 @@ THREE.ParticleSystem = function ( geometry, materials ) {
 
 	this.geometry = geometry;
 	this.materials = materials instanceof Array ? materials : [ materials ];
+	
+	this.sortParticles = false;
 
 };