Future plans for the graphics code

A low level graphics API abstraction

(This is based on the discussion in #1323)

Our current graphics code uses a rather high-level graphics API abstraction which keeps a lot of the graphics code within the OpenGL implementation. The materials system improved this tremendously by unifying how the outside code interacts with the graphics code but this could still be improved a bit by adding a more low-level graphics abstraction.

A possible implementation of this idea could be to use an abstract class for representing a graphics device and use that in the non-abstract graphics code for doing common operations. The graphics device would then implement the necessary operations for the chosen graphics API without having to worry about FSO specific requirements.