ThePythonator

With all the changes I've made, it's got a bit out of sync. The Colour -> Pen and support for PicoSystem are the main commits which I still need, is...

Most of the PicoSystem stuff is done, but audio stuff messes everything up a bit. Using one shared Audio.hpp header is probably the best.

A quick fix (as suggested by @Daft-Freak ) would be to not allow `adsr_end_frame` to be 0: `adsr_end_frame = std::max(uint32_t(1), adsr_end_frame);`