Implementation of missing built-in functions is a separate concern from
outputting the intermediate tree itself as HLSL, so it makes sense to
have all of the built-in emulation in a class that is separate from
OutputHLSL. Being able to reuse the same logic for different emulated
functions also makes the code more compact.

Change-Id: Id503dc3a5c5e743ec65722add56d6ba216a03a7f
Reviewed-on: https://chromium-review.googlesource.com/239872Reviewed-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Nicolas Capens <capn@chromium.org>
Reviewed-by: Olli Etuaho <oetuaho@nvidia.com>
Tested-by: Olli Etuaho <oetuaho@nvidia.com>