glapi: skip padding in get_called_parameter_string

This bug is currently benign, since get_called_parameter_string() is
currently only used for functions that return true for
glx_function.has_different_protocol(), and none of those functions
include padding.  However, in order to implement marshalling of GL API
functions, we'll need to use get_called_parameter_string() far more
often.

Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>

src/mapi/glapi/gen/gl_XML.py

@@ -802,6 +802,8 @@ class gl_function( gl_item ):
         comma = ""
 
         for p in self.parameterIterator():
+            if p.is_padding:
+                continue
             p_string = p_string + comma + p.name
             comma = ", "