ngl: Avoid offscreens in more cases

ngl: Avoid offscreens in more cases

We avoid an offscreen if we know the child node
can 'handle' the transform. Shadow nodes can if their
child node does - either the child node is a text node
in which case the shortcuts we take for shadow nodes
will work fine with the transform (we just render the
text node offset), or the child is not a text node,
in which case we render the shadow to an offscreen
anyway.

This change makes the label-shadows reftest pass with
the GL renderer, not by fixing the issue but by avoiding
it.