don't update window when minimized (#898)

parallel-rdp/interface.cpp

@@ -645,6 +645,9 @@ void rdp_render_frame() {
 }
 
 void rdp_update_screen() {
+  if (SDL_GetWindowFlags(window) & SDL_WINDOW_MINIMIZED) {
+    return;
+  }
   wsi->end_frame();
   wsi->begin_frame();
 }