EEVEE: Fix attributes node on Alpha Clip/Hashed materials

This was cause by a missing implementation of some post
processing attribute functions. Leading to unresolved reference.

source/blender/draw/engines/eevee/shaders/prepass_frag.glsl

@@ -91,3 +91,17 @@ void main()
   }
 #endif
 }
+
+/* Passthrough. */
+float attr_load_temperature_post(float attr)
+{
+  return attr;
+}
+vec4 attr_load_color_post(vec4 attr)
+{
+  return attr;
+}
+vec4 attr_load_uniform(vec4 attr, const uint attr_hash)
+{
+  return attr;
+}