expand canvas to account for drop shadow

When I went to implement a "glow" for my circular progress bar I noticed that the drop shadow was getting clipped at both the top and bottom as well as the left and right extremes of the bar which was a less than ideal situation.  This patch expands the canvas to support the full width of the drop shadow.
This doesn't take care of `shadowOffsetY` and of `options.intial`, also `this.options.shadowBlur` is not always setted.