Don't defer platform backingstore creation if we already have a platform window

The creation was made lazy in 18f415e46d, for those platforms (macOS) that
need a platform window to successfully create a platform backingstore. But we
don't need to delay creation if we actually have a platform window at the time
of constructing the QBackingStore.

Change-Id: I6367736ddca82900dec2751a85a8bc35cc742bb5
Reviewed-by: Volker Hilsheimer <volker.hilsheimer@qt.io>
Reviewed-by: Lars Knoll <lars.knoll@qt.io>
Reviewed-by: Timur Pocheptsov <timur.pocheptsov@qt.io>

src/gui/painting/qbackingstore.cpp

@@ -95,6 +95,11 @@ public:
 QBackingStore::QBackingStore(QWindow *window)
     : d_ptr(new QBackingStorePrivate(window))
 {
+    if (window->handle()) {
+        // Create platform backingstore up front if we have a platform window,
+        // otherwise delay the creation until absolutely necessary.
+        handle();
+    }
 }
 
 /*!