Reinstate back face picking

If enabled and front face picking fails, try points reverse order.
Handling pick results appropriately in unit tests.

Don’t use the triangle bounding volume and ray casting service to
avoid large amount of unnecessary object creation and data copying.

Task-number: QTBUG-57876
Change-Id: I4c4f570cb8de1fa33fcde117f1543e8625633817
Reviewed-by: Sean Harmer <sean.harmer@kdab.com>