Fix Cycles MNEE not working for Metal

Move MNEE to own kernel, separate from shader ray-tracing. This does introduce
the limitation that a shader can't use both MNEE and AO/bevel, but that seems
like the better trade-off for now.

We can experiment with bigger kernel organization changes later.

Differential Revision: https://developer.blender.org/D15070

tests/python/cycles_render_tests.py

@@ -33,8 +33,6 @@ BLACKLIST_OPTIX = [
 ]
 
 BLACKLIST_METAL = [
-    # No MNEE for Metal currently
-    "underwater_caustics.blend",
 ]
 
 BLACKLIST_GPU = [