Refactor Canvas2D inheritance

Canvas2D was inheriting QCanvasPainterItem directly. This caused an
issue where FINAL properties of QCanvasPainterItem and QQuickRhiItem
can cause problems if Canvas2D user adds properties with same names.
Also, some features (colorBufferFormat, effectiveColorBufferSize
etc.) might not make sense for Canvas2D.

So inherit Canvas2D now from QQuickItem instead and have
QCanvasPainterItem as a member variable.

Pick-to: 6.12
Task-number: QTBUG-150707
Change-Id: Ia048223407c92c63fc557ce3d44b3fcfa12a920f
Reviewed-by: Laszlo Agocs <laszlo.agocs@qt.io>