CMakeLists.txt: Include headers/source files in Engine/ External/ recursively.
GraphicsSystem: Add post-render callback.
Add GeometryDrawer class.
GraphicsSystem: Add getter for SDL_Renderer.
main(): Add initialization of GeometryDrawer.
GraphicsSystem: Make background color dark blue instead of white.
Add SDL2_gfx library.
GeometryDrawer: Fix bug where erase could be called with an invalid iterator.