Fix color blending for analytic shadows

Change-Id: I74e8c385a87a3f397b2df53679fcfd9120d82954
Reviewed-on: https://skia-review.googlesource.com/15645
Commit-Queue: Jim Van Verth <jvanverth@google.com>
Reviewed-by: Greg Daniel <egdaniel@google.com>
3 files changed