Fallback to msfilter property if background-size is not supported.

In IE the roundel is much larger than the desired 24x24px, turned out it was because we used the CSS3 property background-size to scale an image, which IE ignores. This fixes it.

Function to test support was required. IE6-8 supports background-size: auto; but not other options such as cover so had to test for ability to handle that.