Finished some of the ToDos

The Mesh, posAccessor and vertexAttribues were only used inside the
scope of their loops and shouldn't have been sitting around in the
functions top-most scope.

For the same reasons as with the attributes, see a few commits prior for
more info.

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.

The name "print_what" is not self-explanatory...

deleted load_mesh function
added documentation

Also added some test files

improved computeModelMatrix

correction in matrix conversion

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

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

materials can now be loaded

At the same time make use of the new UNDEFINED value for error
detection.

materials are not yet included, textures are still necessary for rendering
gltf with 3 meshes as test file (Szene.gltf)