Adding full support for custom materials to GltfLoader

theMinka:

Minor flaw: To add a new ExtensionLoader, one only provides its class rather than an instance. The manager then instantiates it using the default constructor. This complicates implementations that require dependencies or configuration. This could be improved by adding an additional method that accepts an ExtensionLoader instance.

This is a necessary life-cycle requirement because asset loaders are instantiated per thread.