Test and fix SkPMFloat rounding.

Test and fix SkPMFloat rounding.

SSE rounds for free (that was a happy accident: they also have a truncating version).
NEON does not, nor obviously the portable code, so they add 0.5 before truncating.

NOPRESUBMIT=true

BUG=skia:

Committed: https://skia.googlesource.com/skia/+/0aebf5d0d3a2aef38a71885c85303583fdeaad57

[mtklein_C, 2015-03-03 15:59:25]

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

[mtklein, 2015-03-03 16:00:23]

The CQ bit was checked by mtklein@google.com

[commit-bot: I haz the power, 2015-03-03 16:01:13]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/974643002/60001

[commit-bot: I haz the power, 2015-03-03 16:01:14]

Note for Reviewers:
The CQ is waiting for an approval.
If you believe that the CL is not ready yet, or if you would like to L-G-T-M
with comments then please uncheck the CQ checkbox.

Waiting for LGTM from valid reviewer(s) till 2015-03-03 22:01 UTC

[commit-bot: I haz the power, 2015-03-03 16:05:15]

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

[commit-bot: I haz the power, 2015-03-03 16:05:16]

Try jobs failed on following builders:
  Build-Win-VS2013-x86-Debug-Trybot on client.skia.compile (JOB_FAILED,
http://build.chromium.org/p/client.skia.compile/builders/Build-Win-VS2013-x86...)

[reed1, 2015-03-03 16:21:15]

lgtm

[mtklein, 2015-03-03 16:35:04]

The CQ bit was checked by mtklein@google.com

[mtklein, 2015-03-03 16:35:05]

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/974643002/#ps80001 (title:
"f")

[commit-bot: I haz the power, 2015-03-03 16:36:09]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/974643002/80001

[commit-bot: I haz the power, 2015-03-03 16:57:11]

Committed patchset #5 (id:80001) as
https://skia.googlesource.com/skia/+/0aebf5d0d3a2aef38a71885c85303583fdeaad57