Invalidate the scenegraph properly in the rendercontrol

Invalidate the scenegraph properly in the rendercontrol

Taking the persistent flags from the QQuickWindow was a bad idea. These
are not applicable to the case when an application drives the scene via
QQuickRenderControl. Once stop() is called, all resources must be
released since the context itself will typically be destroyed afterwards.

This is a backport of 0b0bb319578bb2e2eced3a80ce3876a4408b72a7 in dev.

On top of this, stop() becomes invalidate() since the original
invalidate() function is never used by anyone. This naming is much cleaner
and is consistent with QSGContext and friends.

Task-number: QTBUG-40505
Task-number: QTBUG-40435
Change-Id: I398f1bda31f335c3508b238f0328305d3cd79ffd
Reviewed-by: Gunnar Sletta <gunnar.sletta@jollamobile.com>