Add shortestArc Quaternion function.
Make DirectionalLights transformable.
Fix lighting with multiple lights.
Refactored fullscreen triangle calculations.
Use explicit null VAO.

No-vertex-attrib draws were working before using the VAO from the model
mesh, but that's not ideal. Added an empty null VAO that is used
instead.
Batching of DirLight renders.

Use instancing to draw all directional lights with a single draw call.