Support ESSL 3.00 EXT_blend_func_extended shaders This adds support for the index layout qualifier that's used in EXT_blend_func_extended to set whether a fragment output should be bound to the primary or secondary blend source color. Output locations are now validated correctly so that two outputs can have the same location as long as they have a different index. Some tests are fixed to allow this. BUG=angleproject:1085 TEST=angle_unittests Change-Id: I1de3ad1406398952287791eca367562bed59d380 Reviewed-on: https://chromium-review.googlesource.com/1245982 Commit-Queue: Olli Etuaho <oetuaho@nvidia.com> Reviewed-by: Geoff Lang <geofflang@chromium.org>