Better heuristic for culling fluid top planes in caves

[muzikbike]

Request Description

This is a requirement if we plan on fixing #2470 at all.

Currently, for a given fluid block at elevation y in the world, the game will check if the block at y+1 and the eight blocks in its Moore neighbourhood are all solid (or contain the same fluid); if so, the interior top face is culled. If not, the face is rendered. This works fine for the most part, but causes oddities in some reasonably common situations.

My preferred solution would be the change proposed in #2472, although it has been stated that such a change may introduce other issues, so a different solution may be required; this will need to be discussed.

For cases like the following, the 16 faces underneath the stone blocks would preferably get culled; whether this also happens to the nine in the middle is a perhaps more controversial issue I may explain in a future report.