Turn off optimizations of qt_depthForFormat on Windows Runtime ARM

This function is apparently optimized in a way that gives a bad return
value (or leaves the variable where it is used uninitialized), leading to
extreme memory allocations and eventual heap exhaustion.

Task-number: QTBUG-42038
Change-Id: Ia4ee9fc6475a0bf40e25eed356b027a4dc68d119
Reviewed-by: Maurice Kalinowski <maurice.kalinowski@theqtcompany.com>

src/gui/image/qimage_p.h

@@ -116,6 +116,9 @@ void qInitImageConversions();
 const uchar *qt_get_bitflip_array();
 Q_GUI_EXPORT void qGamma_correct_back_to_linear_cs(QImage *image);
 
+#if defined(Q_OS_WINRT) && defined(_M_ARM) // QTBUG-42038
+#pragma optimize("", off)
+#endif
 inline int qt_depthForFormat(QImage::Format format)
 {
     int depth = 0;
@@ -158,6 +161,9 @@ inline int qt_depthForFormat(QImage::Format format)
     }
     return depth;
 }
+#if defined(Q_OS_WINRT) && defined(_M_ARM)
+#pragma optimize("", on)
+#endif
 
 QT_END_NAMESPACE