Add float-to-half (binary16) conversion functions.

Add float-to-half (binary16) conversion functions.

Based on code by Fabian Giesen at
https://fgiesen.wordpress.com/2012/03/28/half-to-float-done-quic/.

These will be needed for creating binary16 textures from floating point data.

BUG=skia:3103
Committed: https://skia.googlesource.com/skia/+/936799204b34e7a2f20ac6c0868058799ceb851e

[jvanverth1, 2014-11-26 20:07:53]

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

[bsalomon, 2014-11-26 20:33:45]

lgtm

[jvanverth1, 2014-11-26 20:53:06]

The CQ bit was checked by jvanverth@google.com

[commit-bot: I haz the power, 2014-11-26 20:54:05]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/760753003/1

[commit-bot: I haz the power, 2014-11-26 20:55:45]

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

[commit-bot: I haz the power, 2014-11-26 20:55:46]

Try jobs failed on following builders:
  Build-Ubuntu13.10-GCC4.8-Arm7-Debug-Android-Trybot on client.skia.compile
(http://build.chromium.org/p/client.skia.compile/builders/Build-Ubuntu13.10-GC...)
  Build-Ubuntu13.10-GCC4.8-x86_64-Release-Trybot on client.skia.compile
(http://build.chromium.org/p/client.skia.compile/builders/Build-Ubuntu13.10-GC...)

[jvanverth1, 2014-11-26 21:05:35]

The CQ bit was checked by jvanverth@google.com

[commit-bot: I haz the power, 2014-11-26 21:06:06]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/760753003/20001

[commit-bot: I haz the power, 2014-11-26 21:16:03]

Committed patchset #2 (id:20001) as
https://skia.googlesource.com/skia/+/936799204b34e7a2f20ac6c0868058799ceb851e