Add mirroring

[casey]

Want to be able to mirror the x and y axis to get symmetrical effects. Useful for the waveform.

Probably:

// x = mirror x axis, true or false
// y = mirror y axis, true or false
mirror(x, y);

But this might also be expressible as a coordinate transform

Should also think carefully if we should just make the waveform mirrored, or add a mirrored waveform.