glsl: Use a separate availability class for image atomic built-ins.
glsl: Use a separate availability class for image atomic built-ins.

These are not part of unextended GLSL ES 3.1.

Reviewed-by: Timothy Arceri <7f85c8176b2576ffafd04ac63cd29ed35ff86cb6@yahoo.com.au>
Reviewed-by: Tapani Pälli <cdcb67a5e49297de94340ca09f25994bd5e7f762@intel.com>

metora/MesaGLSLCompiler,metora/MesaGLSLCompiler,metora/MesaGLSLCompiler