use Sk4f to increase radial gradient precision but same or better speed

use Sk4f to increase radial gradient precision but same or better speed

need to land SK_SUPPORT_LEGACY_RADIAL_GRADIENT_SQRT in chrome first

For benchmarks, I timed two cases: unclamped and clamped. The new code happens to treat both as clamped.

On SSE, the new code is faster (and prettier):
BEFORE    383/670
AFTER     276/276

On neon, the new code is slower (but still prettier)
BEFORE    1.44/2.02
AFTER     2.38/2.38

Hoping to find a way to speed things up on neon, to make the decision to take this easier.

BUG=skia:427508

[reed1, 2015-04-23 19:05:46]

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

[reed1, 2015-04-23 19:05:46]

mike, is there a faster way to extract the 4 ints from the 4 floats (the floats
are already scaled, so just (int) is fine)?

[reed1, 2015-04-23 19:06:30]

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

[commit-bot: I haz the power, 2015-04-23 19:06:47]

Dry run: CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1072303005/1

[commit-bot: I haz the power, 2015-04-23 19:12:11]

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

[commit-bot: I haz the power, 2015-04-23 19:12:12]

Dry run: This issue passed the CQ dry run.

[robertphillips, 2015-04-23 19:15:45]

lgtm

[mtklein, 2015-04-23 22:48:10]

On 2015/04/23 19:05:46, reed1 wrote:
> mike, is there a faster way to extract the 4 ints from the 4 floats (the
floats
> are already scaled, so just (int) is fine)?

Yep, we can do 4x float -> 4x int quickly.

[reed2, 2015-04-24 00:16:40]

On 2015/04/23 22:48:10, mtklein wrote:
> On 2015/04/23 19:05:46, reed1 wrote:
> > mike, is there a faster way to extract the 4 ints from the 4 floats (the
> floats
> > are already scaled, so just (int) is fine)?
> 
> Yep, we can do 4x float -> 4x int quickly.

Yum.

Also, I *think* I should be scaling by 256 (or 255.999) instead of 255.

Anyway, I won't try to land this until we try your faster ints (and probably my
new scaling).