Merge 3D Tiles to mesh

[DevMagicLord]

Have an option to turn 3D tiles grid to a mesh, there is multiple advantages :

• more performance when the level is big
• navmesh can be baked on the mesh
• Global illumination will work on mesh
• mesh can be exported
• mesh can be modified with CGG

[TheFlamyy]

Currently it's possible to get the GridMap as (a) baked mesh(es) via GridMap.get_bake_meshes(). With the ResourceSaver this mesh can be saved. Thus this is nothing new!

[DevMagicLord]

It needs a button in the editor for usability, on the grid properties : "Grid to Mesh"

[Calinou]

more performance when the level is big

Note that merging multiple meshes into a single mesh prevents any culling opportunities, which will more often than not decrease performance.

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