Place all instances of a particular component in contiguous memory: I.e., a
scene holds many std::vectors, one for each type of component. These vectors are
looped through every frame. This should optimise things by improving the memory
layout of the program, significantly reducing cache misses.

At some point, add game controller support. Make sure it works well with the
'Input' class.

For mesh rendering, give every mesh-renderer a ShaderMaterial which, depending
on the shader, defines how the mesh reacts to light and also stores a reference
to its texture(s). -- Also make a model loader that works with multiple meshes
(by creating many objects).

Add support for shadows and other complex lighting. Also add post-processing.

For font rendering, put all ASCII characters in one large texture and use
'instancing' (and uniform buffer objects?) to reduce draw calls.

# VULKAN #
The entire vulkan backend needs redesigning without so many classes