Adds an attenuation parameter to corners that corrects the over-coverage
from linear interpolation.
Adds a GM for shared corners that ensures we're doing this right.
Bug: skia:
Change-Id: Iff8bd40554f9fda2e7e03faa3c9fbefe65f27568
Reviewed-on: https://skia-review.googlesource.com/114272
Reviewed-by: Greg Daniel <egdaniel@google.com>
Commit-Queue: Chris Dalton <csmartdalton@google.com>
