Added frustum culling also for ParticleSystems.

This is optional, for now by default it's off, like it was before. It's up to application to update geometry bounding sphere if vertex positions change (like for meshes).

src/objects/ParticleSystem.js

@@ -11,6 +11,22 @@ THREE.ParticleSystem = function ( geometry, material ) {
 
 	this.sortParticles = false;
 
+	if ( this.geometry ) {
+
+		// calc bound radius
+
+		if( !this.geometry.boundingSphere ) {
+
+			this.geometry.computeBoundingSphere();
+
+		}
+
+		this.boundRadius = geometry.boundingSphere.radius;
+
+	}
+
+	this.frustumCulled = false;
+
 };
 
 THREE.ParticleSystem.prototype = new THREE.Object3D();