Use NV_draw_buffers to implement EXT_draw_buffers

Use NV_draw_buffers to implement EXT_draw_buffers

NV_draw_buffers can be used in combination with GLES3.0 to emulate
EXT_draw_buffers. When GLES3.0 and NV_draw_buffers are present, but
native EXT_draw_buffers is not, the extension directive in shaders is
changed in ANGLE to make ESSL 1.00 shaders able to refer to
glFragData[n>0], and DrawBuffersEXT calls are directed to GLES3.0 core
DrawBuffers.

This enables the WEBGL_draw_buffers extension on Tegra K1. Performance
benefits are in the 5-25% range in demos that are able to render both
with and without the WebGL extension.

BUG=421882
TEST=WebGL conformance tests, gpu_unittests

Committed: https://crrev.com/0decd4403598013cb4313b120ad09b3674d7c4c7
Cr-Commit-Position: refs/heads/master@{#299272}

[oetuaho-nv, 2014-10-09 15:05:18]

oetuaho@nvidia.com changed reviewers:
+ bajones@chromium.org, kbr@chromium.org, zmo@chromium.org

[oetuaho-nv, 2014-10-09 15:05:19]

Please review!

[Zhenyao Mo, 2014-10-09 17:51:32]

https://codereview.chromium.org/640283003/diff/1/ui/gl/generate_bindings.py
File ui/gl/generate_bindings.py (right):

https://codereview.chromium.org/640283003/diff/1/ui/gl/generate_bindings.py#n...
ui/gl/generate_bindings.py:219: 'names': ['glDrawBuffersARB',
'glDrawBuffersEXT', 'glDrawBuffers'],
I thought the NV extension function is glDrawBuffersNV? That's not defined in
any header files in chromium code base.  How does it magically map from
glDrawBuffersNV to glDrawBuffers?

[oetuaho-nv, 2014-10-10 07:42:15]

https://codereview.chromium.org/640283003/diff/1/ui/gl/generate_bindings.py
File ui/gl/generate_bindings.py (right):

https://codereview.chromium.org/640283003/diff/1/ui/gl/generate_bindings.py#n...
ui/gl/generate_bindings.py:219: 'names': ['glDrawBuffersARB',
'glDrawBuffersEXT', 'glDrawBuffers'],
On 2014/10/09 17:51:32, Zhenyao Mo wrote:
> I thought the NV extension function is glDrawBuffersNV? That's not defined in
> any header files in chromium code base.  How does it magically map from
> glDrawBuffersNV to glDrawBuffers?

The intention is not to use glDrawBuffersNV, which has somewhat different
behavior, but to use glDrawBuffers from GLES3.0. NV_draw_buffers is used only to
enable ESSL 1.0 shaders to reference multiple render targets, the rest will work
according to GLES3.0 API, where DrawBuffers works the same way as in
EXT_draw_buffers.

Should I maybe add more explanation on this, maybe as a comment in
feature_info.cc?

[oetuaho-nv, 2014-10-10 13:50:20]

I went ahead and added an explanatory comment in patch set 2. I tried to be as
clear as I could, though there's a degree of complexity involved.

[Zhenyao Mo, 2014-10-10 16:13:17]

Thanks for the explanation and the added documentation.

LGTM

[Ken Russell (switch to Gerrit), 2014-10-10 22:21:43]

LGTM though I'm not an OWNER.

[oetuaho-nv, 2014-10-13 07:45:46]

The CQ bit was checked by oetuaho@nvidia.com

[commit-bot: I haz the power, 2014-10-13 07:46:07]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/640283003/50001

[commit-bot: I haz the power, 2014-10-13 08:40:57]

Committed patchset #2 (id:50001)

[commit-bot: I haz the power, 2014-10-13 08:41:46]

Patchset 2 (id:??) landed as
https://crrev.com/0decd4403598013cb4313b120ad09b3674d7c4c7
Cr-Commit-Position: refs/heads/master@{#299272}