Make aurora more adaptable to different display sizes by making boid count adjustable

The sm3.1-teensy3.6-64x64 introduces a bunch of nice improvements

One further thing that can help is to define the number of boids based on the screen size and make sure the demos use not more than said number. See comment on
https://github.com/pixelmatix/aurora/commit/8df7706ea41ba228870dd97e98073662c7e13c52#commitcomment-33333639