glsl: Introduce a new "const_in" variable mode.

This annotation is for an "in" function parameter for which it is only legal
to pass constant expressions.  The only known example of this, currently,
is the textureOffset functions.

This should never be used for globals.

src/glsl/ir_function.cpp

@@ -123,6 +123,7 @@ parameter_lists_match(const exec_list *list_a, const exec_list *list_b)
 	 assert(0);
 	 return -1;
 
+      case ir_var_const_in:
       case ir_var_in:
 	 score = type_compare(param->type, actual->type);
 	 break;