More graceful choosing of EGL configs.

More graceful choosing of EGL configs.

If someone asks for a depth buffer of size 32, we should try to give
them a smaller depth buffer if none of that size is available. Also,
don't unconditionally read the alpha size of the  configure
attributes, since it might have been removed when reducing the config.

Change-Id: If2f9d49c1cc3ded015384f9551b19cd15c523ce3
Reviewed-on: http://codereview.qt.nokia.com/2269
Reviewed-by: Qt Sanity Bot <qt_sanity_bot@ovi.com>
Reviewed-by: Jørgen Lind <jorgen.lind@nokia.com>