Initialize input manager dynamically

To simulate multitouch, we will need more fields in input_manager, which
will be initialized dynamically.

app/src/input_manager.h

@@ -16,6 +16,12 @@ struct input_manager {
     struct screen *screen;
 };
 
+void
+input_manager_init(struct input_manager *input_manager,
+                   struct controller *controller,
+                   struct video_buffer *video_buffer,
+                   struct screen *screen);
+
 void
 input_manager_process_text_input(struct input_manager *input_manager,
                                  const SDL_TextInputEvent *event);