Support running Qt apps on the integrated GPU

By default starting an app that uses OpenGL will
switch machines with dual graphics hardware over
to the discrete GPU. This causes a delay at startup
and possibly increased power usage.

It is possible to prevent the switch:

1) Add the NSOpenGLPFAAllowOfflineRenderers attribute
to the OpenGL pixel format. This is done in this commit.

2) Add <key>NSSupportsAutomaticGraphicsSwitching</key><true/>
to the apps Info.plist. This final opt-in is left to
the app author.

Change-Id: Ibff56fd7770a6b7f8041c8bb60bf290eed24330a
Reviewed-by: Sean Harmer <sean.harmer@kdab.com>
Reviewed-by: Gabriel de Dietrich <gabriel.dedietrich@digia.com>

src/platformsupport/cglconvenience/cglconvenience.mm

@@ -121,6 +121,7 @@ void *qcgl_createNSOpenGLPixelFormat(const QSurfaceFormat &format)
               << NSOpenGLPFASamples << (NSOpenGLPixelFormatAttribute) format.samples();
     }
 
+    attrs << NSOpenGLPFAAllowOfflineRenderers;
     attrs << 0;
 
     NSOpenGLPixelFormat* pixelFormat = [[NSOpenGLPixelFormat alloc] initWithAttributes:attrs.constData()];