Sk4px: alphas() and Load[24]Alphas()

Sk4px: alphas() and Load[24]Alphas()

alphas() extracts the 4 alphas from an existing Sk4px as another Sk4px.
LoadNAlphas() constructs an Sk4px from N packed alphas.

In both cases, we end up with 4x repeated alphas aligned with their pixels.

alphas()
  A0 R0 G0 B0  A1 R1 G1 B1  A2 R2 G2 B2  A3 R3 G3 B3
  ->
  A0 A0 A0 A0  A1 A1 A1 A1  A2 A2 A2 A2  A3 A3 A3 A3

Load4Alphas()
  A0 A1 A2 A3
  ->
  A0 A0 A0 A0  A1 A1 A1 A1  A2 A2 A2 A2  A3 A3 A3 A3

Load2Alphas()
  A0 A1
  ->
  A0 A0 A0 A0  A1 A1 A1 A1  0 0 0 0  0 0 0 0

This is a 5-10% speedup for AA on Intel, and wash on ARM.
AA is still mostly dominated by the final lerp.

alphas() isn't used yet, but it's similar enough to Load[24]Alphas()
that it was easier to write all at once.

BUG=skia:

Committed: https://skia.googlesource.com/skia/+/8a90edc2a58a4f8a4b4da73eb08e943be09538c0

[mtklein, 2015-05-13 13:58:40]

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

[commit-bot: I haz the power, 2015-05-13 13:58:48]

Dry run: CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1138333003/1

[commit-bot: I haz the power, 2015-05-13 14:05:22]

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

[commit-bot: I haz the power, 2015-05-13 14:05:22]

Dry run: This issue passed the CQ dry run.

[mtklein, 2015-05-13 18:17:02]

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

[commit-bot: I haz the power, 2015-05-13 18:17:18]

Dry run: CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1138333003/160001

[mtklein, 2015-05-13 18:34:57]

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

[commit-bot: I haz the power, 2015-05-13 18:40:51]

Dry run: CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1138333003/190001

[mtklein_C, 2015-05-13 18:49:50]

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

[commit-bot: I haz the power, 2015-05-13 19:00:15]

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

[commit-bot: I haz the power, 2015-05-13 19:00:16]

Dry run: This issue passed the CQ dry run.

[reed1, 2015-05-13 19:03:37]

lgtm

[mtklein, 2015-05-13 19:19:13]

The CQ bit was checked by mtklein@google.com

[commit-bot: I haz the power, 2015-05-13 19:19:27]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1138333003/190001

[commit-bot: I haz the power, 2015-05-13 19:19:53]

Committed patchset #11 (id:190001) as
https://skia.googlesource.com/skia/+/8a90edc2a58a4f8a4b4da73eb08e943be09538c0