The UI codebase was suffering with complexity due to the need to
manually convert between two different coordinate systems:
- Top-down "screen coordinates" used by SDL for input and windowing.
- Bottom-up "GL coordinates" used by low-level renderers.
This was making layout calculations diffucult and is bug prone.
With this commit, I'm introducing a 'UIRenderer' abstraction layer that
wraps the low-level 'ShapeRenderer' and 'TextRenderer'. This is
responsible for centralising all coordinate system conversations.
All UI components has been refactored to use the 'UIRenderer' so the
entire UI code opeates exclusively in a single, top-down screen
coordinate system as one would expect.
