Move Sk4px Xfermode code to a header so we can use it twice.

Move Sk4px Xfermode code to a header so we can use it twice.

  - Once in SkXfermode as usual to pick up compile-time SSE and NEON
  - Once in SkXfermode_arm_neon to pick up run-time NEON

This allows us to start cleaning up SkXfermode_arm_neon as we've done
for SkXfermode_SSE2.  I'm saving this catharsis for a day when I need it.

The Sk4px xfermodes are generally faster than the existing NEON procs,
so this should also have the side effect of a perf win there.

This means our new Plus-AA code works for runtime NEON too.
BUG=skia:3852
Committed: https://skia.googlesource.com/skia/+/27c2b09374bb97dfc3a0ab270490209747b91a64

[mtklein_C, 2015-05-22 17:36:08]

mtklein@chromium.org changed reviewers:
+ fmalita@chromium.org

[mtklein_C, 2015-05-22 17:36:14]

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

[commit-bot: I haz the power, 2015-05-22 17:36:29]

Dry run: CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1150313003/20001

[commit-bot: I haz the power, 2015-05-22 17:42:35]

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

[commit-bot: I haz the power, 2015-05-22 17:42:36]

Dry run: This issue passed the CQ dry run.

[f(malita), 2015-05-22 17:53:29]

LGTM

[mtklein, 2015-05-22 17:53:52]

The CQ bit was checked by mtklein@google.com

[commit-bot: I haz the power, 2015-05-22 17:54:05]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1150313003/20001

[commit-bot: I haz the power, 2015-05-22 17:54:44]

Committed patchset #2 (id:20001) as
https://skia.googlesource.com/skia/+/27c2b09374bb97dfc3a0ab270490209747b91a64