glsl: Make it possible to disable a variable in the symbol table.

In later patches, we'll use this in order to implement the required
behaviour that after the gl_PerVertex interface block has been
redeclared, only members of the redeclared interface block may be
used.

v2: Update the function name and comment to clarify that we aren't
actually removing the variable from the symbol table, just disabling
it.

Reviewed-by: Ian Romanick <ian.d.romanick@intel.com>

src/glsl/glsl_symbol_table.cpp

@@ -256,3 +256,18 @@ symbol_table_entry *glsl_symbol_table::get_entry(const char *name)
    return (symbol_table_entry *)
       _mesa_symbol_table_find_symbol(table, -1, name);
 }
+
+void
+glsl_symbol_table::disable_variable(const char *name)
+{
+   /* Ideally we would remove the variable's entry from the symbol table, but
+    * that would be difficult.  Fortunately, since this is only used for
+    * built-in variables, it won't be possible for the shader to re-introduce
+    * the variable later, so all we really need to do is to make sure that
+    * further attempts to access it using get_variable() will return NULL.
+    */
+   symbol_table_entry *entry = get_entry(name);
+   if (entry != NULL) {
+      entry->v = NULL;
+   }
+}

src/glsl/glsl_symbol_table.h

@@ -121,6 +121,14 @@ public:
                                   enum ir_variable_mode mode);
    /*@}*/
 
+   /**
+    * Disable a previously-added variable so that it no longer appears to be
+    * in the symbol table.  This is necessary when gl_PerVertex is redeclared,
+    * to ensure that previously-available built-in variables are no longer
+    * available.
+    */
+   void disable_variable(const char *name);
+
 private:
    symbol_table_entry *get_entry(const char *name);