Handle window resizing in screen

Only the screen knows what to do when the window is resized.

This paves the way to do other actions on window resizing.

app/src/screen.c

@@ -278,6 +278,11 @@ screen_update_frame(struct screen *screen, struct video_buffer *vb) {
     return true;
 }
 
+void
+screen_window_resized(struct screen *screen) {
+    screen_render(screen);
+}
+
 void
 screen_render(struct screen *screen) {
     SDL_RenderClear(screen->renderer);