Z-Sort marks

[lsh]

Currently marks are unsorted in the Z-direction. Reference the implementaitno of visit in vega-scenegraph for a better understanding.

[lsh]

After looking through the way Vega does it it seems like maybe I should make a buffer for items with zindex and a buffer for items without zindex.

[lsh]

An alternative (and cheaper) solution is to sort the z using a the z value in [[builtin(position)]] (probably multiplied by an epsilon).