Add cglContextForContext to QCocoaNativeInterface.

Add cglContextForContext to QCocoaNativeInterface.

This change enables receiving the native CGLContextObj that is used
by a QOpenGLContext. This clearly is non-public api that is only meant to
be used as a last resort for cases where it is really necessary to
get hold of a native context object.

Change-Id: Id00efc88a73d7df04a68c022f19d9d1c4f6d386b
Reviewed-by: Simon Hausmann <simon.hausmann@digia.com>
Reviewed-by: Samuel Rødal <samuel.rodal@digia.com>
Reviewed-by: Morten Johan Sørvig <morten.sorvig@digia.com>