UI block layers: renderOrder does not play well with rest of scene

[lojjic]

The UIBlockLayer3DFacade impl for backgrounds and borders relies on renderOrder to ensure correct transparency blending in deep UI trees. This doesn't play well with other objects in the scene, however, requiring the author to manually set renderOrder for any other objects that may overlap in front of or behind the UI panes.

Let's try to find a better way to handle background/border layering that allows z-sorting to still work like normal.

[lojjic]

The approach I want to try is to force all block layer and text nodes within a flex layout to be given origins with x=0 and y=0, differing only by z according to their node depth. Then they will all have internal transforms shifting their vertices to the correct x/y plane positions and subtracting the origin's z offset to put them back in the same visual plane.

This should ensure that Three's painter sort function will render them back-to-front without using renderOrder.