Report 2.x scale factor for UIView on visionOS

The docs claim that this is the default, but in practice it's not
the case.

Change-Id: I5f6184cbfded2efedbac88023644d3c1e80e9901
Reviewed-by: Timur Pocheptsov <timur.pocheptsov@qt.io>
(cherry picked from commit a05f5457ca033214c1efcf528b85a09a9c758920)
Reviewed-by: Qt Cherry-pick Bot <cherrypick_bot@qt-project.org>

src/plugins/platforms/ios/qiosscreen.mm

@@ -322,7 +322,10 @@ QDpi QIOSScreen::logicalBaseDpi() const
 qreal QIOSScreen::devicePixelRatio() const
 {
 #if defined(Q_OS_VISIONOS)
-    return 2.0; // Based on what iPad app reports
+    // Based on what iPad app reports, and what Apple
+    // documents to be the default scale factor on
+    // visionOS, and the minimum scale for assets.
+    return 2.0;
 #else
     return [m_uiScreen scale];
 #endif

src/plugins/platforms/ios/quiview.mm

@@ -139,6 +139,14 @@ inline ulong getTimeStamp(UIEvent *event)
             };
         }
 #endif
+
+#if defined(Q_OS_VISIONOS)
+        // Although the "Drawing sharp layer-based content in visionOS" docs
+        // claim that by default a CALayer rasterizes at a 2x scale this does
+        // not seem to be the case in practice. So we explicitly set the view's
+        // scale factor based on the screen, where we hard-code it to 2.0.
+        self.contentScaleFactor = self.platformWindow->screen()->devicePixelRatio();
+#endif
     }
 
     return self;