particlparticle-swarm: Use a fixed size timestep

This updates the simulation in fixed size intervals, effectively
decoupling the simulation and rendering loops.
Build with -O0 -g
particle-engine: Update to new Cogl primitive API
particle-swarm: Random starting velocity for boids
web/boids.js: Rename 'turnRate' -> 'agility'
particle-swarm: Implement particle agility
particle-swarm: Add particle size property
particle-swarm: Speed limit is determined by particle size