SVGs can have the same presentation attribute declared redundantly in
both the node attributes and `<style>` tag.
This wouldn't break anything, but we can shave off a few more bytes by
dropping the attribute in this case.
This is just initial thoughts. Feel free to suggest any changes.
Here are described only the new visitor plugins api. Old "full", "perItem", "perItemReverse" will be removed in v3. Old JSAPI class on every node will be also removed so nodes could be plain objects.
