fix the problem that sometimes the body been rendered in the wrong position in ie11

I don't know whether to add two temporary variables to store the values of "2 * hw + 2 + (2 * styles.lineWidth|0)" and "2 * hh + 2 + (2 * styles.lineWidth|0)",then assign them to hiddenCanvas.width,hiddenCanvas.height,view.width and view.height for improving a little performance.
Merge pull request #24 from icemoegirl/patch-3

fix the async loading of datauri images that results in a width = height = 0 on IE 11