Added emulated indexed pointsprite rendering support. This emulation is for renderers that do not support Geometry Shaders. BUG=angleproject:949 Change-Id: I7acf003e83ea6661f10a703486e6d07eb28786f8 Reviewed-on: https://chromium-review.googlesource.com/274851 Reviewed-by: Jamie Madill <jmadill@chromium.org> Tested-by: Cooper Partin <coopp@microsoft.com>