Fix alpha textures in NV ES3 contexts on Windows

Fix alpha textures in NV ES3 contexts on Windows.

Make unit tests iterate over all the rendering GL context types rather than using kNative.

Fix the extension printing when gStartupSpew is set.

Committed: https://skia.googlesource.com/skia/+/e904c09a3a9c701e8d91f2f6ee161feda7615d90

[bsalomon, 2014-07-16 20:25:13]

This is prep work for moving from a single gpu config to gl and gles configs.
The NV change is necessary to make the unit tests pass when they run on the gles
context on my windows box.

[jvanverth1, 2014-07-17 15:28:26]

lgtm

[bsalomon, 2014-07-17 15:46:40]

The CQ bit was checked by bsalomon@google.com

[commit-bot: I haz the power, 2014-07-17 15:47:11]

CQ is trying da patch. Follow status at
 https://skia-tree-status.appspot.com/cq/bsalomon@google.com/398183002/40001

[commit-bot: I haz the power, 2014-07-17 16:03:06]

FYI, CQ is re-trying this CL (attempt #1).
The failing builders are:
  Build-Ubuntu13.10-GCC4.8-x86_64-Release-Trybot on tryserver.skia
(http://108.170.220.76:10117/builders/Build-Ubuntu13.10-GCC4.8-x86_64-Release-...)

[commit-bot: I haz the power, 2014-07-17 16:07:25]

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

[commit-bot: I haz the power, 2014-07-17 16:07:26]

Try jobs failed on following builders:
  Build-Ubuntu13.10-GCC4.8-x86_64-Release-Trybot on tryserver.skia
(http://108.170.220.76:10117/builders/Build-Ubuntu13.10-GCC4.8-x86_64-Release-...)

[bsalomon, 2014-07-17 17:39:09]

The CQ bit was checked by bsalomon@google.com

[commit-bot: I haz the power, 2014-07-17 17:39:37]

CQ is trying da patch. Follow status at
 https://skia-tree-status.appspot.com/cq/bsalomon@google.com/398183002/60001

[commit-bot: I haz the power, 2014-07-17 17:51:05]

Change committed as e904c09a3a9c701e8d91f2f6ee161feda7615d90