onDotClicked() doesn't work when the canvas has padding

[brazzy]

For my game's layout I wanted to give its canvas some padding via CSS, but that caused the coordinate calculation in CanvasIOManager.prototype._listenForMouseClick to fail. I guess the result of getBoundingClientRect() includes the padding. Hopefully there is an easy fix.

While we're at it: there must be a more efficient way to find the correct dot than to iterate over all of them.

I'd be willing to fix this myself, if pull requests are appreciated.