Fix variable mixup from e8f64be6

Just got this warning:

../OpenFlipper/libs_required/ACG/GL/globjects.cc:643:51: warning: variable 'gltex' is uninitialized when used within its own initialization [-Wuninitialized]
      QImage gltex = ACG::Util::convertToGLFormat(gltex);

Introduced by a recent commit that mixed up two variables:

-      QImage gltex = QGLWidget::convertToGLFormat ( qtex );
+      QImage gltex = ACG::Util::convertToGLFormat(gltex);

I didn't test this change, but it seems clear enough that change was unintended.