glsl: allow image*Shadow keywords on ES and GLSL >= 420
These were not reserved keywords in GLSL ES and also allowed on desktop GLSL after 420. New CTS compiler tests will test this.
https://gitlab.khronos.org/Tracker/vk-gl-cts/-/issues/3007
Signed-off-by: Tapani Pälli tapani.palli@intel.com