Add function to convert from window coordinates to viewport

include/window/window.h

@@ -2,6 +2,7 @@
 #define WINDOW_H
 
 #include "c_cpp_aliases/aliases.h"
+#include "vector/vec.h"
 #include <SDL2/SDL_video.h>
 #include <stdbool.h>
 
@@ -30,4 +31,6 @@ void clear_window(window_t *wnd, colour_t colour);
 void set_pixel(window_t *wnd, i32 x, i32 y, colour_t colour);
 void swap_buffers(window_t *wnd);
 
+vec3f_t window_to_viewport(window_t *wnd, i32 x, i32 y, vec3f_t viewport);
+
 #endif // !WINDOW_H