Add ES 3.1 support

[davidrogers-unity]

No description provided.

[cpbotha]

Is there any more information about this work? Is this ES 3.1 support patch being used anywhere?

[aras-p]

I've somehow never merged this patch, but quite possibly ES3.1 might come as a byproduct if I do merge with upstream Mesa one of these days. But right now I don't have much time to do that...

[cpbotha]

Thanks very much for responding!

We've just cherry-picked the ES31 patch into current master, fixed a bunch of merge conflicts, and then merged in #91 to get it all to build cleanly on Linux.

Will report back here as soon as we've tested this on real ES31 shader code.

[aras-p]

Seems to be a really old PR, and I forgot why I have not merged it int yet (also, no tests to know if it works). And would conflict with current master branch. @davidrogers-unity please reopen if still relevant.