Support Base64 embedding of mesh files in URDF and SDF

[johnwason]

Right now it is necessary to store mesh data in separate files from URDF or SDF, but this can cause problems locating the files. HTML allows for embedding data in the URL using "data" URLs and Base64 encoding:

https://developer.mozilla.org/en-US/docs/Web/HTTP/Basics_of_HTTP/Data_URIs

It would make sense to support something that can load Base64 mesh files embedded in the URDF or SDF file. Using the URL field may not be a good idea since most implementations do not expect a large amount of data to be stored there. Maybe add another tag that can be used for Base64 encoded data? I am testing glTF 2.0 binary files, which contain all data for a model in a single archive file (*.glb), including mesh, textures, etc, so it is a good candidate for this use.

[johnwason]

After studying the mesh files more, it looks like it would be fairly easy to forego using external file formats and just store the mesh data inside of the SDF and URDF files. We would need to include:

• Vertex points
• Face indices
• Vertex normals
• Material color and PBR parameters (metallic, roughness)
• Texture images and UV maps

That should be all that is necessary. I am working on loading the additional data from collada and glTF files. Storing the mesh data in SDF or URDF XML would be a simple extension.

[johnwason]

See pull request #490