Remove hardcoded <canvas> element id in CircleDrawer constructor

A canvas id is passed to the `CircleDrawer` constructor in demo.js, but it's not respected in `CircleDrawer`'s code. 
So I replaced the hardcoded `view` string in `CircleDrawer`'s constructor with the `canvasId` argument.

Now the user can freely change the canvas id in his code and the CircleDrawer should work correctly.