Disallow structs as scalar/vector constructor arguments The spec isn't very explicit about disallowing this, but conversions from structs are not among the conversion constructors or specified in any other way either. BUG=angleproject:2036 TEST=angle_unittests Change-Id: I23f2ceda1d1348cec0d3bba38a7a013275ff84eb Reviewed-on: https://chromium-review.googlesource.com/514002 Commit-Queue: Corentin Wallez <cwallez@chromium.org> Reviewed-by: Jamie Madill <jmadill@chromium.org> Reviewed-by: Corentin Wallez <cwallez@chromium.org>