Shared textures are not reused

[gkjohnson]

Textures are always newly loaded when processing a link material, even if it is used and has already been loaded for another visual link node:

urdf-loaders/javascript/URDFLoader.js

Line 539 in f6a7052

material.map = this._textureloader.load(filePath);

Should add a map of loaded textures for every load and / or add an override-able defaultTextureLoader function similar to defaultMeshLoader function so textures can be cached globally if needed.