Prevent initial clipping in Context2D

Don't set an initial clip path and only start clipping once a clip
path has manually been set.

Task-number: QTBUG-39114
Change-Id: Id277775d6eb0be87bead0e5d076f32a07ebdfe5c
Reviewed-by: Gunnar Sletta <gunnar.sletta@jollamobile.com>
Fix crash in QSGRenderContext::invalidate().

QSGRenderContext was holding pointers to QFontEngines without
increasing the font engine's ref count, allowing them to be deleted
before QSGRenderContext could make use of them.

Task-number: QTBUG-36573
Task-number: QTBUG-38313
Change-Id: I0ed28bb44882c55f330c27c23b533b7999d7e04b
Reviewed-by: Gunnar Sletta <gunnar.sletta@jollamobile.com>