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'.
diff --git a/src/glsl/ir_variable.cpp b/src/glsl/ir_variable.cpp
index 4593c18..700c5de 100644
--- a/src/glsl/ir_variable.cpp
+++ b/src/glsl/ir_variable.cpp
@@ -39,9 +39,8 @@
 	     const glsl_type *type, exec_list *instructions,
 		     glsl_symbol_table *symtab)
 {
-   ir_variable *var = new(symtab) ir_variable(type, name);
+   ir_variable *var = new(symtab) ir_variable(type, name, mode);
 
-   var->mode = mode;
    switch (var->mode) {
    case ir_var_auto:
       var->read_only = true;