Fix material assignments being shifted if one of the surfaces has no vertex

meshers/blocky/voxel_mesher_blocky.cpp

@@ -388,6 +388,10 @@ void VoxelMesherBlocky::build(VoxelMesher::Output &output, const VoxelBuffer &bu
 			}
 
 			output.surfaces.push_back(mesh_arrays);
+
+		} else {
+			// Empty
+			output.surfaces.push_back(Array());
 		}
 	}
 

meshers/voxel_mesher.h

@@ -8,6 +8,7 @@ class VoxelMesher : public Reference {
 	GDCLASS(VoxelMesher, Reference)
 public:
 	struct Output {
+		// Each surface correspond to a different material
 		Vector<Array> surfaces;
 		Mesh::PrimitiveType primitive_type;
 	};