Fix buggy blend modes.

Fix buggy blend modes.

The problem turns out to be not over- or underflow like I thought, but that I used different math for x*y/255 for alphas and colors, sometimes resulting in colors where alpha was one less than the maximum color component, which is not a valid SkPMColor.

To be safe, I've switched over all four of these similar complex modes to use exact math everywhere.  They now match the byte-by-byte code in SkXfermode.cpp exactly.

This will slow down Darken and Lighten by about 2x.  I plan to follow up with a CL to see if I can eek out some speed there, and another CL to add asserts that Sk4px code always writes valid SkPMColors.

BUG=skia:4052
Committed: https://skia.googlesource.com/skia/+/44cf62c79ec06f2d9ba408db593f3da2400d3a43

[mtklein_C, 2015-07-14 17:57:20]

The CQ bit was checked by mtklein@chromium.org to run a CQ dry run

[commit-bot: I haz the power, 2015-07-14 17:57:40]

Dry run: CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1241683003/1

[commit-bot: I haz the power, 2015-07-14 18:15:37]

The CQ bit was unchecked by commit-bot@chromium.org

[commit-bot: I haz the power, 2015-07-14 18:15:38]

Dry run: This issue passed the CQ dry run.

[mtklein_C, 2015-07-14 19:18:36]

mtklein@chromium.org changed reviewers:
+ fmalita@chromium.org, reed@google.com

[f(malita), 2015-07-14 20:21:47]

LGTM

These are guarded in Chromium, right?

[mtklein, 2015-07-14 20:37:26]

On 2015/07/14 20:21:47, f(malita) wrote:
> LGTM
> 
> These are guarded in Chromium, right?

Yep.

[mtklein, 2015-07-14 20:37:43]

The CQ bit was checked by mtklein@google.com

[commit-bot: I haz the power, 2015-07-14 20:37:52]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1241683003/1

[commit-bot: I haz the power, 2015-07-14 20:38:33]

Committed patchset #1 (id:1) as
https://skia.googlesource.com/skia/+/44cf62c79ec06f2d9ba408db593f3da2400d3a43