Introduce multiple renderer support

[tulpenkiste]

Draft, VERY WIP

Todo:

• Rendering
  • Move SDL2 code into SDL2 specific backend
  • Preliminary SDL3 renderer
  • Null/headless backend (should be paired with null audio)
• Move event handling and input detection into video backend
• Change renderer to be used via code (deletes all textures)
• CMake rework, statically link one SDL backend
• Proper testing