Add ability to specify a material assignment to a mesh asset.

This extends both the spec and the XML definition to allow specification of a material name on a mesh. This material acts as a fallback in the case that the referencing geom element does not specify a material of it's own.

PiperOrigin-RevId: 798232378
Change-Id: Iaf300c727732744ea1b613e64334ee505ac6e209

src/user/user_mesh.cc

@@ -278,6 +278,7 @@ void mjCMesh::PointToLocal() {
   spec.userface = &spec_face_;
   spec.usertexcoord = &spec_texcoord_;
   spec.userfacetexcoord = &spec_facetexcoord_;
+  spec.material = &spec_material_;
   spec.plugin.plugin_name = &plugin_name;
   spec.plugin.name = &plugin_instance_name;
   spec.info = &info;
@@ -317,6 +318,7 @@ void mjCMesh::CopyFromSpec() {
   content_type_ = spec_content_type_;
   normal_ = spec_normal_;
   face_ = spec_face_;
+  material_ = spec_material_;
   ProcessVertices(spec_vert_);
   texcoord_ = spec_texcoord_;
   facetexcoord_ = spec_facetexcoord_;
@@ -476,6 +478,7 @@ void mjCMesh::CacheMesh(mjCCache* cache, const mjResource* resource) {
   mesh->polygon_map_ = polygon_map_;
   mesh->surface_ = surface_;
   mesh->volume_ = volume_;
+  mesh->material_ = material_;
   std::copy(boxsz_, boxsz_ + 3, mesh->boxsz_);
   std::copy(aamm_, aamm_ + 6, mesh->aamm_);
   std::copy(pos_, pos_ + 3, mesh->pos_);