Render Volumetric Fog at Lower Resolution

## What problem does this solve or what need does it fill?

Using volumetrics is very costly, especially with higher step counts, which seem to be required for a good effect.

## What solution would you like?

Allow rendering only the fog at half or quarter resolution. As far as I can tell this is currently not possible.

## What alternative(s) have you considered?

Maybe volumetrics can be optimized in other ways, but I'm not sure how.

## Additional context


In the scene above with step_count=256, only 86 fps are achieved @ approx 3440x1440 on a 7900xtx.

edit: with `PresentMode::Immediate` and then resizing the window a few times, sometimes I get ~150 fps (which is a bit odd), but still, rendering the fog at a lower resolution would still likely yield significant performance benefits.

Start by locating the volumetric fog rendering entry point and how step_count=256 is applied; the issue names no file or test. Review the PresentMode::Immediate observation separately, then define validation around rendering fog at half or quarter resolution and measuring the resulting performance in a comparable scene.

Written by the indexing model from the issue text.