The list of extension records at the specified offset
in the specified widget_class will be searched for a match on the specified
type, a version greater than or equal to the specified version, and a
record size greater than or equal the specified record_size if it is nonzero.
XtGetClassExtension returns a pointer to a matching extension record
or NULL if no match is found. The returned extension record is owned by
the widget class and must not be modified or freed by the caller, except
possibly to be modified by the widget class owner.