app: add always_on_top

It is very convenient when I play mobile game and watch video at the
same time.

Tested on Linux mint Cinnamon as well as Windows 10.

Signed-off-by: Yu-Chen Lin <npes87184@gmail.com>

app/src/screen.h

@@ -43,7 +43,8 @@ void screen_init(struct screen *screen);
 // initialize screen, create window, renderer and texture (window is hidden)
 SDL_bool screen_init_rendering(struct screen *screen,
                                const char *device_name,
-                               struct size frame_size);
+                               struct size frame_size,
+                               SDL_bool always_on_top);
 
 // show the window
 void screen_show_window(struct screen *screen);