fix TriColorShader to respect the paint's alpha

results can be seen in new gm: vertices_80

BUG=skia:
R=scroggo@google.com

Author: reed@google.com

Review URL: https://codereview.chromium.org/270023002

git-svn-id: http://skia.googlecode.com/svn/trunk/include@14581 2bbb7eff-a529-9590-31e7-b0007b416f81
1 file changed