Vulkan: VoxelGI does not take MultiMeshes into account for baking

[Calinou]

Related to #56027 which is for LightmapGI.

Godot version

4.0.dev (1cbf394)

System information

Fedora 34, GeForce GTX 1080 (NVIDIA 470.74)

Issue description

Similar to #45313, but with the new voxel GI implementation in master.

VoxelGI does not take MultiMeshes into account for baking. On the screenshot below, you can see that objects on the left are smoothly shaded by GI. On the right, it looks very flat due to those objects not being present in the bake:

Left: Individual MeshInstances, right: MultiMeshInstance:

The VoxelGI Lighting debug draw mode also evidences that MultiMeshes are simply not included in the VoxelGI bake, despite having the Baked global illumination mode:

MultiMeshInstances are taken into account by SDFGI however, as demonstrated by the screenshots below:

SDFGI Cascades debug draw mode:

Steps to reproduce

• Add a MeshInstance node with a BoxMesh to act as a floor. Resize it so it can fit several objects on top. Set the global illumination mode to Baked in the inspector.
• Add a MeshInstance node with a BoxMesh to act as a source mesh for the MultiMesh. Set the global illumination mode to Baked in the inspector.
• Add a MultiMeshInstance node. Use the menu at the top of the 3D editor viewport to populate the MultiMeshInstance with BoxMeshes.
• Add a VoxelGI node, select it and click Bake at the top of the 3D editor viewport.

Minimal reproduction project

test_voxelgi_multimesh.zip