Got the TrailBehaviour working but I realised that this is a really
clunky way of doing things.

But it can be quite useful for limited situations.
For example, its madness to use on Medusa --  a bit of code inside
Medusa had to be modified in order to use it correctly.

This leads to an important idea though of auto-refactoring the code,
adding methods or such that are needed to get one of the 'drop-ins' to
work. 

So even though it doesn't work well for Medusa, it still produces a
strange, cool effect.

Instead, I think all creatures should simply be drawn onto their own
PGraphics layers, and opacity/cloning can happen with them to achieve
trail effects.