Fix alpha computation for specular lighting filter (GPU path).

Fix alpha computation for specular lighting filter (GPU path).
Change lighting GM background to show blending.

NOTE:  This will require new results for the lighting GM.

Committed: https://code.google.com/p/skia/source/detail?r=6181

[Stephen White, 2012-10-29 19:29:28]

PTAL

[bsalomon, 2012-10-29 19:36:32]

On 2012/10/29 19:29:28, Stephen White wrote:
> PTAL

LGTM