Fixes infinite reqursion when calling copyTexSubImage2D.

copyTexSubImage2D method was calling itself rather than the
intended glCopyTexSubImage2D method.

Change-Id: I3252dc0ea786c5c6d0d2be17a4ab593f29fad0e9
Task-number: QTBUG-45967
Reviewed-by: Tomi Korpipää <tomi.korpipaa@theqtcompany.com>
Reviewed-by: Pasi Keränen <pasi.keranen@digia.com>