When a URDF link has multiple visual tags, use appropriate material for each

[ANogin]

Currently, when loading a URDF robot, rviz uses the same material for each link, even when the link has multiple <visual> tags with different materials. See #1293 (comment) for a detailed analysis.

Fixes #1293.

I tested that this fixes my use case on ROS Iron (display of a robot inside MoveIt plugin), but do not know enough to test more broadly. The code change appears to apply cleanly to rolling, but I have not tested outside of Iron either. Leaving it to maintainer to figure out the right branch(es) to merge this into.

[ANogin]

I tried applying the same change to the jazzy branch of rviz, and testing with Jazzy. Everything seems to work and look correctly for my use case, but at the same time I am getting a bunch of

[ERROR] [1737532284.422022713] [rviz2]: Can't assign material Shape122Material to the ManualObject MeshShape_ManualObject0 because this Material does not exist in group General. Have you forgotten to define it in a .material script?
[ERROR] [1737532284.426383440] [rviz2]: Can't assign material Shape123Material to the ManualObject MeshShape_ManualObject1 because this Material does not exist in group General. Have you forgotten to define it in a .material script?
[ERROR] [1737532284.430191205] [rviz2]: Can't assign material Shape124Material to the ManualObject MeshShape_ManualObject2 because this Material does not exist in group General. Have you forgotten to define it in a .material script?

type messages. I am guessing this is not necessarily related to my changes, and is likely to be related to #1220 instead, but I am not sure...