Paint each cell exactly once?

[JoshKarpel]

Right now in paint_layout(), we paint every element, then use dictionary merging to layer them on top of each other. This is wasteful, since we can use z-layers and element ordering to determine which element ultimately paints each cell - i.e., a map of cell -> element that will paint that cell. We can then ask that element to paint just that cell.

This might be an optimization for very complex layouts, because we should end up painting fewer cells total (just each cell exactly once instead of some cells many times and then being overridden).

This is pretty bad now that we can paint content too - it means that the amount of painting tends to scale with the area of the divs, not just their surface area.