Avoid rendering invisible faces of simple nodeboxes

[x2048]

This PR adds basic infrastructure to render partial cuboids and adds optimizations for simple adjacent nodeboxes of the same type (snow, slabs, stairs etc.)

Fixes #6409.

To do

This PR is Ready for Review.

How to test

Scenario 1

• Create a V7 world in MTG or an MCL2 world
• Start the game and find a snowy biome with 'slab' snow
• Use noclip or wireframe to see through the blocks
• Notice that occluded snow faces are not rendered

Scenario 2

• Place slabs in MTG in various configurations and combined with solid blocks
• Use noclip or wireframe mode to see through the blocks
• Observe that faces touching solid blocks or adjacent slabs are not rendered.

[sfan5]

Tested and works. This is a good start and we can almost merge this immediately.

I think it's worth expanding on this approach instead of directly going for a (more complicated) generic duplicate vertex removal algorithm.
I was thinking of something like this:

1. The engine precalculates the area covered by each node on each side.
   I suppose an AABB would work here as an approximation but strictly speaking it needs to be multiple.
2. Whenever a box is rendered that touches a node border, the code can look up whether the face is occluded by a (larger or equal size) neighboring face.

[x2048]

I agree with the direction, and I have some ideas about how to optimize internal faces that could be placed in a new PR.