Fix crash with AnimatedImage caused by race condition.

In QDeclarativePixmap::setImage() we deleted a QDeclPixmapData
and recreated a new one in a very short timespan and the new
texture factory was the same pointer as the deleted one, yet a
queued destroyed signal was still emitted. Depending on when the
queued connection was handled in the rendering thread, this would
cause problems with the value returned from textureForFactory.

Change-Id: Ibd785ca12667c99efb88b92689ae7ac4fa87c7ee
Reviewed-by: Kim M. Kalland <kim.kalland@nokia.com>