Pango Bug 602408 - Invalid C++ code breaks compile with Sun C++ Compiler
(Error: A union member cannot have a user-defined assignment operator)

According to the bug:

C++ Programming Language by Bjarne Stroustrup: Chapter 10.4.12 forbids
explicitly using of union members with constructors, destructors or assignment
operations.

So we use a set() method instead of the assignment operator.  Ugly, but hey,
that's life.

We can only reuse the ligid if it belongs to a previous ligature, not a
component!

We expect buffer to be setup with default positions before GPOS.