Update fallback rsqrt implementation to use optimal constants.

Update fallback rsqrt implementation to use optimal constants.

Improves max relative error from 0.00175126 to 0.000650197.
Also add unit tests to check error bounds.

BUG=chromium:511458
Committed: https://skia.googlesource.com/skia/+/29c69793f0201a5f221d6e0f3d41c1adbc4e5656

[jvanverth1, 2015-07-23 17:14:14]

jvanverth@google.com changed reviewers:
+ mtklein@google.com

[jvanverth1, 2015-07-23 17:16:26]

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

[commit-bot: I haz the power, 2015-07-23 17:16:36]

Dry run: CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1251423002/1
View timeline at
 https://chromium-cq-status.appspot.com/patch-timeline/1251423002/1

[mtklein, 2015-07-23 17:18:37]

lgtm

[commit-bot: I haz the power, 2015-07-23 18:00:25]

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

[commit-bot: I haz the power, 2015-07-23 18:00:26]

Dry run: This issue passed the CQ dry run.

[jvanverth1, 2015-07-23 18:10:07]

The CQ bit was checked by jvanverth@google.com

[commit-bot: I haz the power, 2015-07-23 18:10:17]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1251423002/1
View timeline at
 https://chromium-cq-status.appspot.com/patch-timeline/1251423002/1

[commit-bot: I haz the power, 2015-07-23 18:10:58]

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

[commit-bot: I haz the power, 2015-07-23 18:10:59]

Try jobs failed on following builders:
  skia_presubmit-Trybot on client.skia.fyi (JOB_FAILED,
http://build.chromium.org/p/client.skia.fyi/builders/skia_presubmit-Trybot/bu...)

[jvanverth1, 2015-07-23 18:12:07]

jvanverth@google.com changed reviewers:
+ bsalomon@google.com

[jvanverth1, 2015-07-23 18:12:08]

Adding bsalomon@ for API approval.

[mtklein, 2015-07-23 18:12:47]

On 2015/07/23 18:12:08, jvanverth1 wrote:
> Adding bsalomon@ for API approval.

(This is the sort of thing we can TBR with a clear conscience.  There's no API
change here.)

[bsalomon, 2015-07-23 18:13:17]

On 2015/07/23 18:12:47, mtklein wrote:
> On 2015/07/23 18:12:08, jvanverth1 wrote:
> > Adding bsalomon@ for API approval.
> 
> (This is the sort of thing we can TBR with a clear conscience.  There's no API
> change here.)

yup, lgtm

[jvanverth1, 2015-07-23 18:13:58]

The CQ bit was checked by jvanverth@google.com

[commit-bot: I haz the power, 2015-07-23 18:14:07]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1251423002/1
View timeline at
 https://chromium-cq-status.appspot.com/patch-timeline/1251423002/1

[commit-bot: I haz the power, 2015-07-23 18:14:31]

Committed patchset #1 (id:1) as
https://skia.googlesource.com/skia/+/29c69793f0201a5f221d6e0f3d41c1adbc4e5656