Fixes vertexAttribPointer type checking.

Fixes QTBUG-45070. Adds the SHORT enumeration
as accepted type value.

Change-Id: Ibd8ddd6c796320ebc233f0b667250536dd013e74
Reviewed-by: Tomi Korpipää <tomi.korpipaa@digia.com>
Reviewed-by: Pasi Keränen <pasi.keranen@digia.com>