Implemented getParameter() and added state checks.

getParameter now supports reading all values as per specification.
Added client side array check, default texture check and
checking of offset and stride requirements as per specification.

Change-Id: Ia04fc78ea741dbc5afbe969fac50d0253750b58a
Reviewed-by: Tomi Korpipää <tomi.korpipaa@digia.com>
Reviewed-by: Pasi Keränen <pasi.keranen@digia.com>