tablet: Show a default cursor when the cursor has not been set

In this context, the cursor will (almost) always be defined as we set it
as soon as an application is bound to it. We need to show the default
cursor if set_cursor hasn't been called yet. The way to do that is to
check whether the serial is still.

src/input.cpp

@@ -1687,7 +1687,7 @@ public:
                 return ret;
             };
             static const auto defaultCursor = createDefaultCursor();
-            if (!tcursor) {
+            if (!tcursor || tcursor->enteredSerial() == 0) {
                 cursor->updateCursor(defaultCursor.image, defaultCursor.hotspot);
                 return;
             }