Plus xfermode using Sk4px.

Plus xfermode using Sk4px.

Xfermode_Plus runs 4-5x faster.

We expect mixed_xfermodes to have a small diff.  This is because kFoldCoverageIntoSrcAlpha was incorrectly set to true.

This implementation handily beats the Sk4f impl, the portable impl, and the existing SSE2 impl.  Reading the SkXfermodes_opts_SSE2.cpp file, I'm pretty confident that we'll be able to beat all SSE2 impls.

I believe this impl will beat or match the existing NEON impl too, but that may not be true for more complicated xfermodes.  They can take advantage of transposing ARGBARGB... to AAAARRRR.... cheaply and I haven't figured out an abstraction for that yet that doesn't screw SSE.

Adds:
  - MapDstSrc() to Sk4px
  - saturatedAdd() to SkNi (only implemented as far as it's used).
  - div255Narrow()

BUG=skia:

Committed: https://skia.googlesource.com/skia/+/6cbf18c70bf99f58b2bb1c49cdf8d41be561fee4

[mtklein, 2015-05-12 14:36:03]

mtklein@google.com changed reviewers:
+ mtklein@google.com, reed@google.com

[mtklein, 2015-05-12 14:36:09]

The CQ bit was checked by mtklein@google.com to run a CQ dry run

[commit-bot: I haz the power, 2015-05-12 14:36:17]

Dry run: CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1138893002/20001

[commit-bot: I haz the power, 2015-05-12 14:42:09]

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

[commit-bot: I haz the power, 2015-05-12 14:42:09]

Dry run: This issue passed the CQ dry run.

[reed1, 2015-05-12 15:45:59]

very pretty -- lets try landing w/ the aa version.

[mtklein, 2015-05-12 18:11:57]

The CQ bit was checked by mtklein@google.com to run a CQ dry run

[commit-bot: I haz the power, 2015-05-12 18:12:11]

Dry run: CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1138893002/70001

[mtklein, 2015-05-12 18:36:07]

The CQ bit was unchecked by mtklein@google.com

[mtklein, 2015-05-12 18:49:27]

The CQ bit was checked by mtklein@google.com to run a CQ dry run

[commit-bot: I haz the power, 2015-05-12 18:50:22]

Dry run: CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1138893002/90001

[commit-bot: I haz the power, 2015-05-12 19:03:22]

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

[commit-bot: I haz the power, 2015-05-12 19:03:23]

Dry run: This issue passed the CQ dry run.

[mtklein, 2015-05-12 19:30:05]

The CQ bit was checked by mtklein@google.com to run a CQ dry run

[commit-bot: I haz the power, 2015-05-12 19:30:32]

Dry run: CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1138893002/110001

[commit-bot: I haz the power, 2015-05-12 19:36:30]

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

[commit-bot: I haz the power, 2015-05-12 19:36:31]

Dry run: This issue passed the CQ dry run.

[reed1, 2015-05-12 20:32:06]

purty -- lgtm (assuming we can guard for blink and eventually test on neon)

[mtklein, 2015-05-12 20:34:41]

The CQ bit was checked by mtklein@google.com to run a CQ dry run

[commit-bot: I haz the power, 2015-05-12 20:34:59]

Dry run: CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1138893002/130001

[commit-bot: I haz the power, 2015-05-12 20:41:02]

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

[commit-bot: I haz the power, 2015-05-12 20:41:03]

Dry run: This issue passed the CQ dry run.

[mtklein, 2015-05-12 20:44:42]

The CQ bit was checked by mtklein@google.com to run a CQ dry run

[mtklein, 2015-05-12 20:44:43]

The patchset sent to the CQ was uploaded after l-g-t-m from reed@google.com
Link to the patchset: https://codereview.chromium.org/1138893002/#ps150001
(title: "guard")

[commit-bot: I haz the power, 2015-05-12 20:44:58]

Dry run: CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1138893002/150001

[reed1, 2015-05-12 20:48:08]

lgtm

[commit-bot: I haz the power, 2015-05-12 20:51:09]

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

[commit-bot: I haz the power, 2015-05-12 20:51:10]

Dry run: This issue passed the CQ dry run.

[mtklein, 2015-05-12 22:47:38]

The CQ bit was checked by mtklein@google.com

[commit-bot: I haz the power, 2015-05-12 22:47:55]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1138893002/150001

[commit-bot: I haz the power, 2015-05-12 22:48:14]

Committed patchset #9 (id:150001) as
https://skia.googlesource.com/skia/+/6cbf18c70bf99f58b2bb1c49cdf8d41be561fee4