Initialize flag memory in raster paint engine to avoid valgrind warning

After a new flag bit was added in 6.4, so that the flags no longer
exactly fills a byte, valgrind will sometimes give a "... depends on
uninitialised value" warning.

Fixes: QTBUG-107649
Pick-to: 6.4
Change-Id: Iaaf5ae7bba3ce2476be5b2277839cad1d18aa55f
Reviewed-by: Christoph Cullmann <cullmann@kde.org>
Reviewed-by: Paul Olav Tvete <paul.tvete@qt.io>

src/gui/painting/qpaintengine_raster.cpp

@@ -516,6 +516,7 @@ QRasterPaintEngineState::QRasterPaintEngineState()
 
     txscale = 1.;
 
+    flag_bits = 0;
     flags.fast_pen = true;
     flags.non_complex_pen = false;
     flags.antialiased = false;