only the first visual of each link has its material resolved

[erwincoumans]

See https://github.com/ros/urdfdom/blob/master/urdf_parser/src/model.cpp#L132
it sets 'the link visual material', but there might be an array of visuals in each link.

It seems only the first link visual gets its material resolved. This could be an issue if those additional links only contain a material with a name, and not RGBA/texture filename information.