Notion of pivot point now for rotating, i.e. the anchor point at which
rotations are applied. Rotating the reticule is done by using inverse
trigonometry, namely atan2f.
Some functions which should not be exposed in the API have been switched
to INTERNAL scope. We have a notion of an empty texture in World Traveller
that we can pass in situations where we just want to render a solid
colour with no associated texture.
The alternative to this was creating a separate shader for rendering
primitives but would require at some point to expose the AssetManager to
the renderer or the user on behalf has to manually switch shaders before
rendering (non-intuitive).
