Drop shadows are typically created using the DropShadowFilter class.
However, the DropShadowFilter, like all bitmap filters,
can be computationally expensive.
If the DropShadowFilter is applied to a DisplayObject,
then the drop shadow is recalculated
whenever the appearance of the object changes.
If the DisplayObject is animated (using a Resize effect, for example),
then the presence of drop shadows hurts the animation refresh rate.
This class optimizes drop shadows for a common case.
If you are applying a drop shadow to a rectangularly-shaped object
whose edges fall on pixel boundaries, then this class should
be used instead of using the DropShadowFilter directly.
This class accepts the first four parameters that are passed
to DropShadowFilter: alpha, angle,
color, and distance.
In addition, this class accepts the corner radii for each of the four
corners of the rectangularly-shaped object that is casting a shadow.
Once those 8 values have been set,
this class pre-computes the drop shadow in an offscreen Bitmap.
When the drawShadow() method is called, pieces of the
precomputed drop shadow are copied onto the passed-in Graphics object.