[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.

• 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.
• inherit_godot_noise: makes noise classes provided by the module inherit Noise, so they can be used in more places (depends on Make Noise virtual so custom noises can be created godotengine/godot#100443)
• mesher_script: WIP, allows to create custom meshers using scripts
• transvoxel_single_material_per_voxel: introduces a new texture mode in the Transvoxel mesher, which interprets each voxel as containing only one material. Simpler to use and faster, but transitions can only be 1-voxel long
• graph_raycast_down: WIP, adds a function to VoxelGeneratorGraph to calculate the height of the highest surface from an SDF generator at specific coordinates
• connected_textures: WIP, introduces the concept of "tiles" to blocky voxel cubes, where sides can connect to neighbor voxels if they share the same tile, or randomize across multiple variants, or extend larger patterns.

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)