Don't re-parent ShaderEffect source items.

Don't re-parent ShaderEffect source items.

ShaderEffect and ShaderEffectSource used to set themselves as
parent of and hide source items that don't already have a parent.
This was done in order to make sure the source items got a canvas.
This patch sets the canvas of the source items without re-parenting.

Task-number: QTBUG-23069
Change-Id: I24d56e6eb970590bca3adff7a61459d25e4983a0
Reviewed-by: Gunnar Sletta <gunnar.sletta@nokia.com>