glsl: Count function call outvals as writing to variables for linker checks.

Fixes: glsl-vs-position-outval.  Bug #28138 (regnum online)

src/glsl/linker.cpp

@@ -106,6 +106,27 @@ public:
       return visit_continue_with_parent;
    }
 
+   virtual ir_visitor_status visit_enter(ir_call *ir)
+   {
+      exec_list_iterator sig_iter = ir->get_callee()->parameters.iterator();
+      foreach_iter(exec_list_iterator, iter, *ir) {
+	 ir_rvalue *param_rval = (ir_rvalue *)iter.get();
+	 ir_variable *sig_param = (ir_variable *)sig_iter.get();
+
+	 if (sig_param->mode == ir_var_out ||
+	     sig_param->mode == ir_var_inout) {
+	    ir_variable *var = param_rval->variable_referenced();
+	    if (var && strcmp(name, var->name) == 0) {
+	       found = true;
+	       return visit_stop;
+	    }
+	 }
+	 sig_iter.next();
+      }
+
+      return visit_continue_with_parent;
+   }
+
    bool variable_found()
    {
       return found;