Decrease the default VoxelGI quality to improve performance

[Calinou]

Follow-up to #49738 and #54158.

On a GeForce GTX 1080 in 2002×1447 resolution, decreasing VoxelGI quality from High to Low quality saves 1.2 ms of GPU time in a medium-sized test scene. This only results in a minor drop in quality.

This helps make the default VoxelGI performance more competitive with SDFGI (to the point where VoxelGI generally surpasses SDFGI now, which wasn't the case on my PC before).

Preview

High quality (current default)

Low quality (new default)

[mrjustaguy]

VoxelGI > SDFGI on lower end hardware (GTX 650 level for example), due to the cascade jumps making it unusable where VoxelGI runs Perfectly fine.
Yeah, SDFGI is good when the camera isn't getting translated, but when it is, it's the lows that matter, not the avg, so it isn't such a simple comparison between the 2

[akien-mga]

Approved in PR review meeting.

[akien-mga]

Thanks!