[client] ds: added new getFullscreen operation

As the window manager may change our mode to full screen without our
request we must ask the ds backend for the current state when we want to
toggle the mode.

client/displayservers/SDL/sdl.c

@@ -452,6 +452,11 @@ static void sdlSetFullscreen(bool fs)
   SDL_SetWindowFullscreen(sdl.window, fs ? SDL_WINDOW_FULLSCREEN_DESKTOP : 0);
 }
 
+static bool sdlGetFullscreen(void)
+{
+  return (SDL_GetWindowFlags(sdl.window) & SDL_WINDOW_FULLSCREEN_DESKTOP) != 0;
+}
+
 struct LG_DisplayServerOps LGDS_SDL =
 {
   .probe               = sdlProbe,
@@ -482,6 +487,7 @@ struct LG_DisplayServerOps LGDS_SDL =
   .wait                = sdlWait,
   .setWindowSize       = sdlSetWindowSize,
   .setFullscreen       = sdlSetFullscreen,
+  .getFullscreen       = sdlGetFullscreen,
 
   /* SDL does not have clipboard support */
   .cbInit    = NULL,