Finished some of the ToDos

The loadScene function has grown out of proportion and needs to be
refactored. As a first step, the parsing of vertex attributes has been
encapsulated into its own function.

This commit also adds the remaining PrimitiveTypes of vertex groups.

With precise documentation about translating the glFW/OpenGL description
of sampler objects to the Vulkan description that we want.

deleted load_mesh function
added documentation

correction in matrix conversion

This commit has breaking changes to the API header of the asset_loader,
meaning things will not compile until the implementation in
asset_loader.cpp is adjusted.

The implementation in asset_loader.cpp is still a hack, as is the usage
in main.cpp, but the hack is now using the textures array of the mesh
and the struct could be cleaned up to get rid of the hack in the modules
interface.

This commit only defines the structs and makes sure that the
construction is still valid, but does not implement the usage of the new
structs.

Changes according to comments in the issue, mainly in .cpp
Changed enum to match fx-gltf in .hpp

Signed-off-by: Tobias Frisch <tfrisch@uni-koblenz.de>

This commit adds the header for the asset loader module containing a
first draft of what the API for loading single meshes will look like.

Nothing is set in stone, all is up for discussion.