Particle rendering
* PP attrs
* shaders to make use of them - specify 'varying float' for per-particle attrs
* custom attr export - RAT ....docs/modeling/artisan/artisan.html
* AppNote #22 - 'primitive variables'
* AppNote #18 - 'Points'
* RAT notes on particle rendering
* RAT - MTOR primitive vars

* sample scene with particles
* Bl.sl , particleRen.sl
* particleRen_1.rib (uses 'Bl' shader), output
* particleRen_2.rib (uses 'particleRen' shader), output