Added a size method to the mesh class. It may have made more sense to have an empty method but eh.
Pulled out the renderable class as it no longer seems to be needed. May have to push it back in later but it's out for now.
Chunk's model_matrix and mesh are now pulled out by reference. I also short circuit intersection testing if _mesh.size() <= 0.
Fixed the primary rendering problem. Turns out that std::pair<glm::ivec3, Chunk> is copied when iterating through an unordered_map. This caused issues with the renderable making references to the model matrix and meshes.