If a DDB is used as a source for an operation that can't be handled
natively by GDI, we end up needing to take a really slow path (creating a
temporary surface for acquire_source) for each operation. If we convert
the DDB to a DIB, we then end up having a real image buffer and can hand
things off to pixman directly.
This isn't the default mode because I'm not sure if there are cases where a
DDB is explicitly needed (e.g. for printing), and it would change
current cairo behaviour. It might become the default at some point in the
future.
