[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/include/interface/displayserver.h

@@ -130,8 +130,9 @@ struct LG_DisplayServerOps
   /* wait for the specified time without blocking UI processing/event loops */
   void (*wait)(unsigned int time);
 
-  /* set the window dimensions */
+  /* get/set the window dimensions */
   void (*setWindowSize)(int x, int y);
+  bool (*getFullscreen)(void);
   void (*setFullscreen)(bool fs);
 
   /* clipboard support */
@@ -162,5 +163,6 @@ struct LG_DisplayServerOps
    (x)->uninhibitIdle      && \
    (x)->wait               && \
    (x)->setWindowSize      && \
-   (x)->setFullscreen)
+   (x)->setFullscreen      && \
+   (x)->getFullscreen)
 #endif