Copy QSGEngine functions to QQuickCanvas.

Long term we intend to remove the QSGEngine class
all together so this is the first step. It duplicates
some of the logic but doesn't break anything.

Also including an example on how to use it in
examples/declarative/openglunderqml

Change-Id: I69ed93ec5fa1b5c4c746169306d38f8d6ce80477
Reviewed-by: Gunnar Sletta <gunnar.sletta@nokia.com>