Teardown view before QQuickRenderControl
Deleting QQuickWindow will force any remaining runnables to complete. This is important for any items (such as our new ThumbnailItem) that deletes resources in queued cleanups.
We need to invoke this manually whilst we still have an active render control with a valid context, otherwise remaining runnables will no-op.
This fixes a memory leak observed with the new Overview effect.