This re-lands EXT_blend_func_extended implementation. A lot of the
implementation has been rewritten on top of changes that have been
done since the last attempt.

For now we only expose the extension on desktop GL. To support GLES,
we'd need to investigate what's the most robust way to handle the
compiler output and make the binding of the fragment outputs
conditional.

The extension is disabled on AMD and Intel because of test failures.

BUG=angleproject:1085
TEST=angle_end2end_tests

Change-Id: I619ae3162769b90aad095ddec158ce6c57a114a8
Reviewed-on: https://chromium-review.googlesource.com/1233713Reviewed-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Geoff Lang <geofflang@chromium.org>
Commit-Queue: Olli Etuaho <oetuaho@nvidia.com>