regulate velocity of flock members
implement flocking behaviour: separation
implement flocking behaviour: velocity alignment
implement flocking behaviour: centering
revert back to using velocity, not heading/speed

turns out it actually made the maths harder...
fix buggy rotation of sprites

the angle calculation was finding the smallest angle, regardless of
direction, where I always wanted it clockwise. This hack emulates the
desired result by inserting a factor of -1 when the angle will be
ensure heading is never initialised as zero
split velocity into speed and heading components

This will make it easier to do direction changes later, as we can keep
the heading normalised without worrying about the speed
don't try to normalize near-zero heading vec
use lyon to generate triangle boids
rotate sprites according to their velocity
gen 10 sprites with random location, velocity, colour
implement Bounds as ResMut that updates per frame

it now reads the actual window dimensions each frame, instead of
declaring a hardcoded const
keep sprite from going off screen
put a moving sprite on the screen
add squirrel entity