Add material library object that can be loaded from an asset pack
- Doesn't yet resolve texture references
Resave all progressive .jpg Sponza textures as non-progressive, so stb_image can read them
Add texture library, which indexes a set of textures from an asset pack by name
- Loading a material library can look up textures from a texture library
- Added lowercasing of all material names and filenames
Mesh loader now looks up material references in a material library
Render Sponza using its actual textures, instead of just applying one texture to everything
Remove D3DX, which is no longer needed now that everything is using stb_image.
Update test-ovr so it renders Sponza with its materials, too.  It can't build the asset pack itself yet though because of relative path issues.