Implement keyboard/mouse control

To control the device from the computer:
 - retrieve mouse and keyboard SDL events;
 - convert them to Android events;
 - serialize them;
 - send them on the same socket used by the video stream (but in the
 opposite direction);
 - deserialize the events on the Android side;
 - inject them using the InputManager.

app/src/convert.h

@@ -0,0 +1,20 @@
+#ifndef CONVERT_H
+#define CONVERT_H
+
+#include <SDL2/SDL_stdinc.h>
+#include <SDL2/SDL_events.h>
+#include "controlevent.h"
+
+// on Android, a scroll event requires the current mouse position
+struct complete_mouse_wheel_event {
+    SDL_MouseWheelEvent *mouse_wheel_event;
+    Sint32 x;
+    Sint32 y;
+};
+
+SDL_bool input_key_from_sdl_to_android(SDL_KeyboardEvent *from, struct control_event *to);
+SDL_bool mouse_button_from_sdl_to_android(SDL_MouseButtonEvent *from, struct control_event *to);
+SDL_bool mouse_motion_from_sdl_to_android(SDL_MouseMotionEvent *from, struct control_event *to);
+SDL_bool mouse_wheel_from_sdl_to_android(struct complete_mouse_wheel_event *from, struct control_event *to);
+
+#endif