Also, fix a three-part hack:

1. rsvg_standard_element_start() would set the new node's parent right
   there, in redundant fashion to where rsvg_node_group_pack() sets the
   node's parent.

2. Make rsvg_standard_element_start() add the new node to its parent
   before node->set_atts() gets called...

3. ... so that rsvg_stop_set_atts() doesn't have to set its own parent
   to ctx->priv->currentnode (!) because the stop's node hasn't been
   parented yet.  This was the only place where a node->parent gets
   overwritten after creation.