Underwater mode for water block height

As noted in #1855: Having all water blocks below blocks have full height might be desirable for underwater world renders. However, this would cause full height water blocks to be shown at the water surface (similar to the image above).
Minecraft works around this by simply not rendering the water surface when viewed from below for such water blocks (no air in the 3x3 blocks above), but we can't have one-sided water surfaces in a path tracer. Maybe a follow-up PR could add an underwater mode where we apply full block height to all water blocks with a non-air block above them.