Addresses [GzWeb] Support particle emitters.

We use ShaderParticleEngine, a third-party library based in ThreeJS to render particles.

I followed this doc to set default values for the Particle Emitter, and to translate them to values used by SPE.

Since SPE has its limitations, I expect some tweaks are going to be necessary as we implement more particles in models.

Some things to have in mind:

Color Range Images cannot be used directly. SPE requires at most four color values to interpolate through the lifetime of the particle. We can have a similar behaviour by reading the rgba values of four pixels in the image.
Particles doesn't seem to be sorted by depth correctly (a particle rendered in a position behind another would appear to be forward, for instance). Playing with depthTest and depthWrite values of SPE didn't worked, so another way to fix this should be needed.
SPE uses just a texture for particles, and we cannot set the diffuse value of the material, for example.

The SPE Library was one of the best ones that I found for this task, but as we implement more particles and face its limitations, we might want to move away from it.

Can you ptal @iche033 or @nkoenig ?

Particle Emitter

GIF of a Fog Emitter

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