fix the problem that world.render sometimes renders body to wrong positi...

...on in ie11

When the 2D context draws a image which isn't loaded,image.width and image.height are zero,then context will draw this image to wrong position.Some browsers like chrome are very fast and the image always has loaded using ctx.drawImage(view, -view.width / 2, -view.height / 2).But ie is so slow that we have to add a explicit onload function.