This previously wasn't done because of fear that it would affect the traditional fixed-sized canvas,
but only items that _layout_sensitive (i.e. are packed into a constraint packer directly) will
actually do anything in ::size_allocate().
Possibly might want to relax this to cover items that have a constraint packer between them
and a root group.
Remove Canvas::Layout, use Canvas::Container for the same purpose, move child-rendering into Item::render_children() so that it
could theoretically be used by any derived type.
