DDS Mipmaps

[ryanw]

I needed support for mipmaps in uncompressed 3D DDS files. So I've added support for that very specific format. It seems to work fine.

I'm not much of a C/C++ dev, so before I bother with 2D + compressed formats, is the way I've implemented it done sensibly? Would a PR like this be accepted?

Diff so far: master...ryanw:dds-mipmaps