(#823) correctly points out that commit
e3bb911b broke geometry computation.
I didn't see how it could, since it didn't even have to do with
extracting bounding boxes from elements.

It turns out that I forgot to copy the DrawingCtx.drawsub_stack for
the nested context.  So, for an SVG with structure

  a
    b
      c -> with isolation context (due to non-100% opacity / etc.)
        d
        e
        f id="draw_only_this_one"
        g

if one calls geometry_for_layer("draw_only_this_one"), draw_tree() would
properly create a drawsub_stack like (a b c f), but then the nested
rendering would go:

  stack_top=a

  stack_top=b

  stack_top=c (creates isolation context, and a nested DrawingCtx)

  stack_top=empty (nested DrawingCtx has an empty drawsub_stack) ->
    draw every child of c since there's no drawsub_stack to limit it,
    instead of just drawing f in there.

GNOME/librsvg#823

Part-of: <GNOME/librsvg!697>