QMdiArea: fix unintended reallocations

The capacity of 'result' is reserved, but QVector::clear() always
deallocates. Fix by using resize(int) instead, which at least
preserves capacity when it has been reserve()d.

Change-Id: I198d532661f4f767588c4ce244c2454eb8823e54
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@theqtcompany.com>

src/widgets/widgets/qmdiarea.cpp

@@ -512,7 +512,7 @@ QVector<QRect> MinOverlapPlacer::findMaxOverlappers(const QRect &domain, const Q
         if (overlap >= maxOverlap || maxOverlap == -1) {
             if (overlap > maxOverlap) {
                 maxOverlap = overlap;
-                result.clear();
+                result.resize(0);
             }
             result << srcRect;
         }