Let's keep source compatibility and define `drop-shadow-blur` to be a radius.
The CSS spec says that the standard deviation is half of the radius.
We just need to scale again and increase the shadow rect to make sure that no borders are visible.
This replaces the box gradient with a new implementation that
behaves like the HTML Canvas element, by applying a gaussian
blur to the alpha of the rectangle to shadow.
The drop-shadow-blur property is changed to be now just a "level", like
the HTML Canvas shadowBlur property, which is defined to be half of the
standard deviation of the gaussian blur to be applied.
