fix TriColorShader to respect the paint's alpha

fix TriColorShader to respect the paint's alpha

results can be seen in new gm: vertices_80

BUG=skia:

Committed: http://code.google.com/p/skia/source/detail?r=14581

[scroggo, 2014-05-05 21:15:02]

lgtm

https://codereview.chromium.org/270023002/diff/1/src/core/SkDraw.cpp
File src/core/SkDraw.cpp (right):

https://codereview.chromium.org/270023002/diff/1/src/core/SkDraw.cpp#newcode2436
src/core/SkDraw.cpp:2436: const int alphaScale = alpha + (alpha >> 7);
From the comments in SkColorPriv, it looks like this is the more accurate
version of SkAlpha255To256; would it make sense to provide both as helpers?

https://codereview.chromium.org/270023002/diff/1/src/core/SkDraw.cpp#newcode2457
src/core/SkDraw.cpp:2457: scale0 = (scale0 * alphaScale) >> 8;
Why not use SkAlphaMul?

[reed1, 2014-05-05 21:25:14]

https://codereview.chromium.org/270023002/diff/1/src/core/SkDraw.cpp
File src/core/SkDraw.cpp (right):

https://codereview.chromium.org/270023002/diff/1/src/core/SkDraw.cpp#newcode2436
src/core/SkDraw.cpp:2436: const int alphaScale = alpha + (alpha >> 7);
On 2014/05/05 21:15:02, scroggo wrote:
> From the comments in SkColorPriv, it looks like this is the more accurate
> version of SkAlpha255To256; would it make sense to provide both as helpers?

Done.

https://codereview.chromium.org/270023002/diff/1/src/core/SkDraw.cpp#newcode2457
src/core/SkDraw.cpp:2457: scale0 = (scale0 * alphaScale) >> 8;
On 2014/05/05 21:15:02, scroggo wrote:
> Why not use SkAlphaMul?

Done.

[scroggo, 2014-05-05 21:27:19]

lgtm

[reed1, 2014-05-05 21:28:06]

The CQ bit was checked by reed@google.com

[commit-bot: I haz the power, 2014-05-05 21:28:21]

CQ is trying da patch. Follow status at
 https://skia-tree-status.appspot.com/cq/reed@google.com/270023002/20001

[commit-bot: I haz the power, 2014-05-05 21:35:20]

Change committed as 14581