[Tracker] Feature branches

[Zylann]

Features have been developped using more branches lately, so this is a tracker to list them.
These branches should be working in their current state but might be incomplete, more buggy than usual, and can get breaking changes. You are welcome to test them or propose some changes if you need what they implement. Documentation about them should be either present on linked issues, or directly in the doc changes of each branch.
They will be merged to the main branch once they are considered stable enough or don't need more breaking changes.

• blocky_fluids: adds a specific kind of implementation of "fluid models" to VoxelMesherBlocky (No issue created; this branch depends on partial_side_culling)
• partial_side_culling: adds the ability for some blocky models to be partially "cutout" by their neighbors instead of being either fully culled or not culled (no issue created, was discussed on Discord)
• navigation: attempt to implement generic navmesh support. In a rough state. See Navmesh for Non-Blocky Voxel (Lod) Terrain #610
• clipbox_deferred_merges: improves the Clipbox LOD streaming system to reduce recently edited chunks flickering when moving fast backwards (no issue created, was discussed on Discord). Not sure if it will be merged given the added complexity. There might be better options.
• dmc2: attempt at re-implementing dual marching cubes, based on the implementation from the FastNoise2 preview tool. It still doesn't implement adaptativity or sharp features, however it has the advantage of producing nicer geometry and uses a simpler approach to hide LOD seams (no special shader required). However voxel materials are not well figured out at the moment, instancing needs adjustments, and detail rendering doesn't work with it.
• instancer_emitters: refactors VoxelInstanceLibrary to contain "emitters" which themselves can contain one or multiple items. Allows to fulfill The ability for VoxelInstancer to choose an array of mesh or scenes to spawn using one VoxelInstanceGenerator #682. In a rough state. Not worked on, until there is real need for it.

Note: a lot of other branches exist, but they are not pending features. Some are version branches. Most others are old merged branches that happen to still exist a so they are listed by Github. Maybe some archiving process should be decided to sort them out (I don't want to delete them)