Fix DEPTH_STENCIL buffers

We can't assume combined depth/stencil buffer is supported,
so check for it, and initialize it correctly when it is
supported.

Change-Id: Id1792f51ef0a6c74437327e6308c88157fcc1124
Task-number: QTBUG-49182
Reviewed-by: Tomi Korpipää <tomi.korpipaa@theqtcompany.com>
Reviewed-by: Pasi Keränen <pasi.keranen@digia.com>