Multi renderer backend support for Brux

[tulpenkiste]

This issue simply exists so I can remember to work on this for brux in the near future.

Renderers on initial completion should probably be:

• SDL2 + SDL2_gfx
• SDL3 + SDL GPU (allows for shaders) (the base SDL3 backend needs to be backwards compatible with SDL2, SDL GPU cannot as thats a new feature to SDL3)

I will probably split this into 2 PRs (first being initial work on making multiple renderers possible, second being SDL3 due to the lack of SDL2_GFX (and also I have to wait for SDL GPU to be merged)) and this will be started after #66 is completed and merged into unstable.