the caller instantiates a light (distant, point or spot), and an
SkDiffuseLightingFilter or SkSpecularLightingImageFilter with that light. A
Sobel edge detection filter is applied to the alpha of the incoming bitmap, and
the result is used as a height map for lighting calculations.
Review URL: http://codereview.appspot.com/6302101/
git-svn-id: http://skia.googlecode.com/svn/trunk@4314 2bbb7eff-a529-9590-31e7-b0007b416f81
