by avoiding making multiple copies of the Primitive2D container
that we pass to TransformPrimitive2D.
Instead, make TransformPrimitive2D store its children using a
GroupPrimitive2D, which means we can share the GroupPrimitive2D
among all the TransformPrimitive2D instances we create.
Change-Id: I8a4398f9db6a6ab013ee24ad53836975fba6f3df
Reviewed-on: https://gerrit.libreoffice.org/c/core/+/162951
Tested-by: Jenkins
Reviewed-by: Noel Grandin <noel.grandin@collabora.co.uk>