No OpenGL stereo support on Mac

Creating a QGLWidget using a QGLFormat with setStereo(true) does not even try to create a stereo OpenGL on Mac OS. And the widget then reports that it does support stereo (w.format().stereo() -> true).

The broken code is

qtbase/src/platformsupport/cglconvenience/cglconvenience.mm

where the routine that chooses the Mac OpenGL pixel format qcgl_createNSOpenGLPixelFormat() simply ignores the stereo setting. The code which records the QGLFormat properties of the actual OpenGL context obtained qtbase/src/plugins/platforms/cocoa/qcocoaglcontext.mm, routine QCocoaGLContext::QCocoaGLContext() also ignores the OpenGL context and simply says the format exactly matches the requested format.

I've provided a patch that fixes the problem. It tries to get a stereo OpenGL context when the QGLFormat requests one, and if stereo is not supported provides a mono OpenGL context. Also the patch code will report the actual properties of the OpenGL context that was obtained.