QCachedPainter: fix pixmapRect() for dpr != 1

Return the correct (device independent) size of the pixmap rect.
This amends 75617efe238bb20e9a0e1d12a1a8bf5d6f6819e1.

Task-number: QTBUG-129680
Change-Id: I19ed4665143824204de77e01ae47772d79fbf0ff
Reviewed-by: Volker Hilsheimer <volker.hilsheimer@qt.io>

src/widgets/styles/qstyle_p.h

@@ -57,7 +57,8 @@ public:
     }
     QRect pixmapRect() const
     {
-        return QRect(0, 0, m_pixmap.width(), m_pixmap.height());
+        const auto sz = m_pixmap.deviceIndependentSize();
+        return QRect(0, 0, sz.width(), sz.height());
     }
     QPainter *operator->()
     {