Implement ParticleGeometryBuffer.

[player-03]

This offers more ways to define particles, and provides some much-needed documentation. (If we'd had this documentation beforehand, I wouldn't have gotten confused and submitted #51.)

Take the basic Particles sample. It creates a ParticleGeometry like so:

var plane:Geometry = new PlaneGeometry(10, 10, 1, 1, false);
var geometrySet:Vector<Geometry> = new Vector<Geometry>();
for (i in 0...20000)
	geometrySet.push(plane);

//...

_particleMesh = new Mesh(ParticleGeometryHelper.generateGeometry(geometrySet), material);

This is the most common use case for particles: a ton of copies of a single thing. So why should you have to allocate a vector of 20000 items? ParticleGeometryBuffer has a function specifically for this, making the process that much easier.

var buffer:ParticleGeometryBuffer = new ParticleGeometryBuffer();
buffer.addParticles(new PlaneGeometry(10, 10, 1, 1, false), 20000);

//...

_particleMesh = new Mesh(buffer.getParticleGeometry(), material);

There's also a function to add particles one at a time if that's more convenient, or you can keep using ParticleGeometryHelper as if nothing changed.

Supersedes #49.