Skybox should render on its own layer, to allow room for an external chaperone overlay

[hypevhs]

This comes from a discussion in the LVRA discord.
lovr-playspace is what FOSS XR folks use to display a SteamVR-like playspace boundary/chaperone.
WlxOverlay-S uses session_layers_placement: 5 for the entire session. This includes foreground panels(screens, keyboard, watch, etc) and the skybox. This leaves no room for the chaperone to be above the skybox, but below the Wlx panels.
If lovr-playspace picks layer <5, the chaperone is (mostly?!) invisible since the skybox renders on top: SpookySkeletons/lovr-playspace#2
If lovr-playspace picks layer >5, the chaperone renders on top of the skybox and the panels, which make it difficult to read the panels.

i think the screens should be in front of the lovr stuff and the skybox behind
but that won't work with just 1 wlx session