The functions are emulated on HLSL, and use the native functions on
OpenGL 4.2+ and GLES3.0. Emulation for OpenGL versions <= 4.1 is not yet
implemented.

BUG=angle:865

Change-Id: I6803a1767dacdb3dca12f13924651fd38fcacb75
Reviewed-on: https://chromium-review.googlesource.com/240961Reviewed-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Geoff Lang <geofflang@chromium.org>
Tested-by: Olli Etuaho <oetuaho@nvidia.com>