Fix canvas item drawImage bug with FBO render target

If canvas' render target is FramebufferObject, only the first drawImage() call
was successful, it could be fixed by adding beginNativePainting() and endNativePainting(),
but this could be unexpected behavior with OpenGL paint engine.

task-number: QTBUG-23595
Change-Id: Iff70fc88f6d39655dd283fa5dde3365887910460
Reviewed-by: Michael Brasser <michael.brasser@nokia.com>