GPU performance investigation with volumetric clouds

[JonnyOThan]

After fixing the resolution issues, the performance is noticeably worse (not surprising). Need to do some profiling to see if there's any obvious easy fixes.

See also LGhassen/EnvironmentalVisualEnhancements#45

[JonnyOThan]

I was doing a little high-level investigation in non-vr mode in my "pretty" install. It seems like IVA mode is MUCH more expensive for some reason (at least during reentry).

57% GPU usage:

IVA mode goes to 71% GPU usage:

This seems like one of those times where a depth prepass would help - but it's not immediately obvious why IVA is so much worse here.