Removed nlohmann-json as a submodule. It's now being grabbed through the fx-gltf submodule.

Added the libktx library as a dependency for the project.
LibKTX is now used as back-end for loading KTX files in AssetManager-system.
Made significant work on texture loading. Still WIP, but uncompressed textures are now supported.
Made signifant work on Renderer::TextureDocument and AssetManager::TextureDocument structures.
Moved functions for tying together AssetManager and Renderer into it's own header and source file.
Modified the mesh shader to support a diffuse texture.

Signed-off-by: Nils Petter Skålerud <np_skalerud@hotmail.com>