Implement loading an image when file is dropped

include/drop_area.h

@@ -2,13 +2,17 @@
 #define DROP_AREA_H
 
 #include "colour.h"
+#include "image.h"
 #include "window.h"
 #include <stdbool.h>
 
 typedef struct {
   render_rect_t rect;
+  render_rect_t image_rect;
   colour_t border_colour;
+  image_t image;
   bool mouseover;
+  bool image_loaded;
 } drop_area_t;
 
 void init_drop_area(window_t *wnd, drop_area_t *area, render_rect_t rect,