Brushes
As of v0.5, we have four brushes (8 counting the inverted color state, 2 thinking in terms of similarity): flat, circle, dot, and fan. These are actually two brushes with minor changes to become four.
From one position to the next, draw a line penSize wide. At very low speeds, this creates a spindly, almost hairy mess, but at high speeds it creates blocks. Coded the same as fan, but the previous stroke updates every event instead of only the initial.
From the start position to all subsequent positions, draw a line `penSize` wide. Coded the same as fan, but the previous stroke is never updated after start.