Refactoring : Fold DisplayInterface into SDLDisplay for the sake of
simplicity.
Refactoring: Rename SDLDisplay source files.
Fix typo in shader name.
Allow tesselation level to be varied using + / - keys.
Tinkering - use a second buffer for colour data and add a second
triangle.
Tweaks / fixes.
Take advantage of implicit rules to simplify make
Distinguish intermediate components.
Create a basic Vector4 class.
Add [] operator and a basic set of unit tests.
Implement Magnitude()
Add Normalize and multiply / divide.