This is needed so that ImagingFactory can have a threading model of Both.

Perhaps interesting to note is that this returns an IDirectDrawSurface instead
of an IDirectDrawSurface4.

This is a pretty naive implementation, should that become a performance
problem it's easy enough to speed up with a search tree of some kind.

This should be handled by the context's draw buffer management.

For example, interpolating palette indices doesn't have the desired result.
Should we really want filtering for these cases we could implement it inside
the relevant shaders, after the fixup, but I doubt it's worth the effort.