Shininess

[AlexandreLamure]

The shininess value is set to 20 by default. We should get the value from the material, using assimp.

[AlexandreLamure]

Good resources:
https://tharindumathew.com/2018/11/07/loading-shininess-in-materials-when-using-assimp/
In particular:

const aiMaterial* ai_material = scene.mMaterials[material_index];
float shininess;
if(AI_SUCCESS != aiGetMaterialFloat(ai_material, AI_MATKEY_SHININESS, &shininess))
{
    shininess = 20.f; // if unsuccessful set a default
}

[AlexandreLamure]

The value should be passed as in, from the VAO of the Mesh class, in setupMesh().
However, this solution breaks the semantics of the Vertex class. An other VAO could be created, but it seems too much pain for a simple float.
An other solution would be to give it as a uniform in draw().
I prefer to let it with a default value to keep the code clean.