This is a simple flocking simulation using the Boids algorithm. Originally
written for 15-462. Could be adapted into a screensaver, if anyone even uses
those anymore.

Controls:
w,s - move goal point into/out of screen
a,d - move goal point left/right
q,e - move goal point up/down
f - randomly select new goal point on surface of sphere
g - toggle follow cam
r - reset simulation
x - toggle integrator (euler (default) -> midpoint -> RK4)

====================================================
Algorithm overview
====================================================

Forces are calculated as follows:
- cohesion: linear with distance to each tile up to CO_DIST
- separation: inverse square of distance to each tile up to SEP_DIST and clamped
  by SEP_MAX
- alignment: the component of the average velocity of the surrounding tiles up
  to ALIGN_DIST that is normal to the current velocity
- goal-seeking: linear toward the goal minus the current velocity

All weights are as specified at the top of tile.c; separation weight increases
with velocity to make the swarm expand as it is moving toward a goal.

Banking is done by rotating the things by the arctangent of the magnitude of the
lateral force over the up-vector; more lateral force means angle of rotation
closer to 90 degrees.

====================================================
Notes
====================================================

Tiles are colored by their velocity for an interesting effect.

Try changing NTILES in tiles.c for different effects.