Added PIXI.canvas global object for fill/clear/px rounding

The new booleans allow for tighter configuration of the canvas renderer. Allows the dev to control if fillRect / clearRect is called (as it's not needed for lots of games and can help perf slightly on legacy devices). Also added PX_ROUND boolean. If true (default is false) it will Math.floor the x/y values on the transform, this stops interpolation when Sprites are rendered at non-pixel values, keeping pixel art crisp. Also helps performance on older mobiles. I removed the view.style.backgroundColor as it was never set, mostly commented-out and is unreliable as hell on Android anyway :( (which I guess is why it had been replaced with the fillRect instead).