Disallow texture lookup functions in global initializers Do this by simply disallowing all function calls in global initializers - all built-in math ops are handled as other ops, not function calls. This change was tested extensively with popular WebGL content, with no regressions found. TEST=angle_unittests BUG=angleproject:988 Change-Id: Id1107fa294ae4012d5dd3949539d0b7b4cd21943 Reviewed-on: https://chromium-review.googlesource.com/283703 Reviewed-by: Zhenyao Mo <zmo@chromium.org> Tested-by: Olli Etuaho <oetuaho@nvidia.com>