Add blend optimization helpers and use to convert rect draws to clears.

Add blend optimization helpers and use to convert rect draws to clears.

Committed: http://code.google.com/p/skia/source/detail?r=10723

[bsalomon, 2013-08-12 18:21:12]

remove GM change from this CL

[bsalomon, 2013-08-12 18:25:40]

This is a rework of https://codereview.chromium.org/21877006/ with bug fixes.

It fixes two issues: 

1) It computes the correct blended color when determining if the paint is
constant color and opaque.

2) Checks whether the rect is stroke.

This CL has the just the updates relative to
https://codereview.chromium.org/21877006/ and is easier to review:

https://codereview.chromium.org/22907002

Feel publish comments to that CL if it is easier.

[jvanverth1, 2013-08-12 19:41:28]

lgtm

[commit-bot: I haz the power, 2013-08-14 18:31:08]

CQ is trying da patch. Follow status at
https://skia-tree-status.appspot.com/cq/bsalomon@google.com/22558003/4001

[commit-bot: I haz the power, 2013-08-14 19:00:24]

Retried try job too often on Build-Mac10.7-Clang-x86-Release-Trybot for step(s)
BuildSkiaLib, BuildTests
http://108.170.217.252:10117/buildstatus?builder=Build-Mac10.7-Clang-x86-Rele...

[commit-bot: I haz the power, 2013-08-14 19:15:19]

CQ is trying da patch. Follow status at
https://skia-tree-status.appspot.com/cq/bsalomon@google.com/22558003/18001

[commit-bot: I haz the power, 2013-08-14 21:56:43]

Change committed as 10723