Add Qt::AA_UseHighDpiPixmaps.

Setting this attribute enables QIcon::pixmap() to return
high-dpi pixmaps when running on "retina" type displays.

This requires an opt-in flag since the returned pixmap
can be larger than the requested size, which is a change
in previous documented behaviour that can break existing
code.

Change-Id: I5ff3d25c68de24aa4eda7ad1f8aa9199da04707e
Reviewed-by: Gabriel de Dietrich <gabriel.dedietrich@digia.com>

src/corelib/global/qnamespace.h

@@ -498,6 +498,7 @@ public:
         AA_X11InitThreads = 10,
         AA_SynthesizeTouchForUnhandledMouseEvents = 11,
         AA_SynthesizeMouseForUnhandledTouchEvents = 12,
+        AA_UseHighDpiPixmaps = 13,
 
         // Add new attributes before this line
         AA_AttributeCount

src/corelib/global/qnamespace.qdoc

@@ -167,6 +167,14 @@
            to left button mouse events instead. This attribute is enabled
            by default.
 
+    \value AA_UseHighDpiPixmaps Make QIcon::pixmap() generate high-dpi pixmaps
+           that can be larger than the requested size. Such pixmaps will have
+           devicePixelRatio set to a value higher than 1.
+
+           After setting this attribute application code that uses pixmap
+           sizes in layout geometry calculations should typically divide by
+           QPixmap::devicePixelRatio() to get device-independent layout geometry.
+
     \omitvalue AA_AttributeCount
 */