Clean up checking variable packing limits This encapsulates expanding struct variables inside the VariablePacker class that packs variables according to the GLSL ES spec. The variable expansion step is no longer run twice when checking uniforms against the max uniforms limit. BUG=angleproject:2068 TEST=angle_unittests Change-Id: I012ddaa249f71c0a78d937c98007c61352e64888 Reviewed-on: https://chromium-review.googlesource.com/608367 Commit-Queue: Olli Etuaho <oetuaho@nvidia.com> Reviewed-by: Jamie Madill <jmadill@chromium.org>