glsl2: Add and use new variable mode ir_var_temporary

This is quite a large patch because breaking it into smaller pieces
would result in the tree being intermitently broken.  The big changes
are:

    * Add the ir_var_temporary variable mode

    * Change the ir_variable constructor to take the mode as a
      parameter and correctly specify the mode for all ir_varables.

    * Change the linker to not cross validate ir_var_temporary
      variables.

    * Change the linker to pull all ir_var_temporary variables from
      global scope into 'main'.

src/glsl/ir_function.cpp

@@ -116,6 +116,7 @@ parameter_lists_match(const exec_list *list_a, const exec_list *list_b)
       switch ((enum ir_variable_mode)(param->mode)) {
       case ir_var_auto:
       case ir_var_uniform:
+      case ir_var_temporary:
 	 /* These are all error conditions.  It is invalid for a parameter to
 	  * a function to be declared as auto (not in, out, or inout) or
 	  * as uniform.