In both, using our allocation directly for the child is wrong; we
should create a new allocation that's our width and height.
In ShellDrawingArea, also need to chain up to parent.
ShellDrawingArea is a size-independent wrapper for a ClutterCairoTexture.
Useful when drawing non-fixed size areas.
ShellStack is a simple container class which holds items
in a completely overlapping Z stack. The main difference
from ClutterGroup is that items will be constrained to
(and allocated) the size of the stack, not getting their
preferred size always.