Support mipmaps for writing nutexb

This should use the existing mipmap data for DDS files with proper alignment per mipmap (512 bytes) and swizzling applied. Swizzling will already apply the appropriate alignment by default. Automatically generating DDS mipmaps is out of scope for this project since compressed formats like BC7 are not unique to nutexb. Allowing manually specified mipmaps for DDS may be a better option and allows sharing the API with uncompressed images.
- [x] dds mipmaps
- [ ] #7

Start by tracing the existing DDS mipmap handling and the nutexb writing path. Check how mipmap data, 512-byte alignment, and swizzling are currently handled, then determine how manually specified mipmaps could share an API with uncompressed images. Done means DDS mipmaps are written with the required alignment and swizzling without automatically generating compressed mipmaps.

Written by the indexing model from the issue text.