glsl: Expose image load and store built-ins in GLSL ES 3.1.
glsl: Expose image load and store built-ins in GLSL ES 3.1.

Reviewed-by: Tapani Pälli <cdcb67a5e49297de94340ca09f25994bd5e7f762@intel.com>

metora/MesaGLSLCompiler,metora/MesaGLSLCompiler,metora/MesaGLSLCompiler