Particle system docs & fixes

Added documentation for particlesystem component.
Old particle system was deleted.
Removed 2 from ParticleEmitter2/particle2.
depthTest renamed to depthWrite in particles (that's what it actually does. needs migration).
Removed unneeded code from simulation.
Fixed flickering bug when using CPU wrapping.

Fixed another gamma correction related problem on particles, replaced old (now unnecessary) slow code in particle fragment shader with a faster one.