Update canvas.js

[lexxai]

Allow use own HTML canvas, by canvas = fx.canvas('canvasid'); <canvas id="canvasid" ....

[petethepig]

+1

[denkan]

Looking for this. However, would rather pass the HTMLCanvasElement as param to fx.canvas(), as I may not have a certain id on the canvas element.

var c = document.getElementById('canvasid'); 

// or I could use this:
// var c = document.getElementsByTagName('canvas')[0];

canvas = fx.canvas(c);