Improve Fluid Shaping

[douira]

Refactors the fluid renderer to improve fluid shaping in a variety of situations. Profiling shows that the performance impact appears to be negligible.

Screenshots are ordered dev and then this patch.

Regular water slopes look the same.

Water slopes are nearly identical. I experimented with changing the sloping behavior and it didn't work out well.

Water shaping around stairs (foreground) and between stairs (further back): Water height doesn't propagate through the closed sides of block faces, fixing issues with unnecessarily bent water faces and water connecting to waterlogged blocks where it shouldn't.
Fixes #1211

The cave heuristic has been refined to search 2 blocks around and 1 block up (compared to 1 block around and 1 block up). This improves the appearance of inward up face culling somewhat. Player builds are still somewhat weird but generally improved.
To make the presentation more self-consistent and to avoid situations where seams in the fluid are visible from air spaces that the player could inhabit, some cases now have more fluid quads than before. In other cases, less faces are shown to make glass rendering more consistent. Glass being right above water doesn't constitute enough "air" to make it render inwards facing quads, but outwards facing quads will render to ensure the fluid is visible when viewing it through the glass from above.
Fixes #2470, nearly completely, to the point where I would call it fixed.
Fixes #2935 by providing a feasible yet pretty good cull heuristic.
Fixes #2947 by hiding down-facing fluid up quads against glass in all reasonable cases.

Outward up faces are now culled with the refined cave heuristic. This slightly increases rendering performance if there's a lot of flooded visible caves. I have reports of increased fps anywhere up to around 2 to 6%.

The test for bottom quad occlusion to show inward water quads on air, but not on glass for example. By using the same test for overlays as the sides, this more consistently shows and hides the inward bottom face.
Fixes #1210
Fixes #1212

Fixes #3523

[douira]

ok I'm marking this as ready, what follows will be fixing any bugs we might find in further testing.

[douira]

I've fixed a small bug with the flooded cave heuristic and added detailed documentation to the PR and the fluid renderer class itself.

[IMS212]

Seems fine to me. Not very knowledgeable about fluid rendering, though.

[douira]

Thanks! I've done a few rounds of testing and feedback. If something significant pops up after release, we can fix it.