rename paint_order to rendering_order

Closes #310 (closed)

cc @Moini

The term "paint order" means something different in SVG.

Alternatives are

• z_order
• stacking_order
• drawing_order
• rendering_order

I ultimately chose rendering_order because that's how the chapter is named in the spec - even though the only sentence in that chapter uses "drawing order". It probably doesn't really matter, but paint_order is indeed confusing.