After this patch, it is possible to set a flag to change
EXT_draw_buffers extension directives to NV_draw_buffers in ESSL.
This enables users of ANGLE to emulate EXT_draw_buffers by using
NV_draw_buffers in combination with GLES3.0.

Change-Id: I5dacdbd6cd0d0362424ea3791557342c42efd4bd
Reviewed-on: https://chromium-review.googlesource.com/219941Reviewed-by: Olli Etuaho <oetuaho@nvidia.com>
Tested-by: Olli Etuaho <oetuaho@nvidia.com>
Reviewed-by: Jamie Madill <jmadill@chromium.org>