This emulation is for renderers that do not support Geometry Shaders.

BUG=angleproject:949

Change-Id: I7acf003e83ea6661f10a703486e6d07eb28786f8
Reviewed-on: https://chromium-review.googlesource.com/274851Reviewed-by: Jamie Madill <jmadill@chromium.org>
Tested-by: Cooper Partin <coopp@microsoft.com>