Create generic buffer type

src/main.c

@@ -36,7 +36,7 @@ int main(void) {
     return TINY_EXIT_MODEL_LOAD_FAILED;
   }
 
-  clear_image(&(render.img), bg);
+  clear_buffer(&(render.img), &bg);
   render_model(&model, &render, teal, RENDER_TYPE_SHADED, COLOUR_TYPE_FIXED);
   save_image(&(render.img), "result.pam");