Enforce texture cleanup to happen at the end of the sync phase.

This was ok for the threaded renderer, but for the single-threaded
renderers where the event loop was spinning we could run the risk
of scheduling a texture for deleteLater() and then cleaning it
up before we got around to the sync phase.

Task-number: QTBUG-35670
Change-Id: I92074294b427fd8ab2a1823ba79c7ac3b59094ec
Reviewed-by: Michael Brasser <michael.brasser@live.com>