Fixes compile issues with mingw.

Original idea to generate unique id's for texture images from class pointer
doesn't compile on mingw, switching to generate the id with a simple
incremental uint value.

Change-Id: Ife3b268103bb28d1ab7722f8d312480e3b79391e
Task-number: QTBUG-45161
Reviewed-by: Pasi Keränen <pasi.keranen@digia.com>