Only switch draw buffers if the extension is supported

Only switch draw buffers if the extension is supported

Otherwise it's possible to end up crashing when calling a null function
pointer, if a framebuffer does not have a color buffer attached.

This bug applies to at least some GLES3 platforms, where DrawBuffers
function is not bound (which is reasonable, since core version of the
function behaves subtly differently than DrawBuffersEXT in relation to
GLSL ES 1.0 shaders, if the EXT_draw_buffers extension is missing). Also
possibly GLES2 platforms which implement OES_depth_texture but not
EXT_draw_buffers.

BUG=401645
TEST=gpu_unittests, WebGL conformance tests

Committed: https://src.chromium.org/viewvc/chrome?view=rev&revision=288347

[oetuaho-nv, 2014-08-07 19:15:06]

Looks like issue in https://codereview.chromium.org/375413003 was never properly
addressed once the commit was reverted. Please review.

[Zhenyao Mo, 2014-08-07 20:32:37]

LGTM

Thank you for pushing this through.

[oetuaho-nv, 2014-08-08 07:03:36]

The CQ bit was checked by oetuaho@nvidia.com

[commit-bot: I haz the power, 2014-08-08 07:04:32]

CQ is trying da patch. Follow status at
 https://chromium-status.appspot.com/cq/oetuaho@nvidia.com/448223002/1

[commit-bot: I haz the power, 2014-08-08 11:32:00]

FYI, CQ is re-trying this CL (attempt #1).
The failing builders are:
  win_chromium_rel on tryserver.chromium.win
(http://build.chromium.org/p/tryserver.chromium.win/builders/win_chromium_rel/...)

[commit-bot: I haz the power, 2014-08-08 14:02:58]

Change committed as 288347