MockCompositor: Protect against race condition

The QList m_fractionalScales was being changed from both the main thread
and the dispatch thread. This lead to read-after-free AddressSanitizer
errors when running autotests. Using a DirectConnection makes sure that
both appending and removal happens in the same thread. This does not
fix a theoretical race condition in DefaultCompositor::fractionalScale().

Fixes: QTBUG-130627
Pick-to: 6.8
Change-Id: Ie73f80d08929bcfcbd9940acdb4f86233df013cb
Reviewed-by: David Edmundson <davidedmundson@kde.org>

tests/auto/wayland/shared/fractionalscalev1.cpp

@@ -16,7 +16,7 @@ void FractionalScaleManager::wp_fractional_scale_manager_v1_get_fractional_scale
     auto *scaler = new FractionalScale(s, resource->client(), id, resource->version());
     connect(scaler, &QObject::destroyed, this, [this, scaler]() {
         m_fractionalScales.removeOne(scaler);
-    });
+    }, Qt::DirectConnection);
     m_fractionalScales << scaler;
 }