Of course we managed to rely on a GLSL feature that is only
in GLSL 130 and newer, not 120 which is what the default 2.1 OpenGL
contexts support on macOS.
Change-Id: Ib75e750ea15d59e51b2207669068fba7719a48b1
Pick-to: 6.6
Reviewed-by: Laszlo Agocs <laszlo.agocs@qt.io>