Add OccluderInstance3D baking

[Calinou]

• Related to Generate UV2 and define a lightmap size hint to allow lightmap baking #15.

Godot's own OccluderInstance3D baker is not able to bake Cyclops nodes into an occluder, since it only accepts MeshInstance3D nodes. This means you can't use Godot's occlusion culling unless you manually place occluders, which is time-consuming.

It should be possible to manually create an ArrayOccluder resource to give to an OccluderInstance3D node. This bypasses Godot's own occluder baker entirely. This can be one occluder per shape, or one for the entire collection of blocks (which may be faster to render).

Additionally, the baked occluders could be shrunk along the normals to avoid overocclusion (similar to gongpha/gdQmapbsp@9d031ad).

[blackears]

I've added code to generate occluders. I'm not sure if it works though - I've not used occluders before and don't know how to test it.

I was also not able to add the ability to shrink the occluder size. This is going to have to wait on blocks being free to be anywhere in the scene tree.