when resolving mesh filenames, convert relative paths to absolute
when resolving mesh filenames, convert relative paths to absolute

fixes #1636

billhoffman/drake,billhoffman/drake,sheim/drake,sheim/drake,billhoffman/drake,billhoffman/drake,sheim/drake,billhoffman/drake,billhoffman/drake,sheim/drake,billhoffman/drake,sheim/drake,billhoffman/drake,sheim/drake,sheim/drake,sheim/drake