Properly support URIs to GLB in Instanced 3D Models

[javagl]

The TileData payload field carries a note that it can either contain the GLB data or an URI.

Functions like glbToI3dm or the (internal) extractGlbBuffers always assume that the payload is an embedded GLB. But this should be generalized. This will include some minor implementations for on-the-fly resolving of the GLB data from the URI and the corresponding error handling. It may also involve extending the CLI. For example, one could consider to offer an option like

... glbToI3dm -i input.glb -o output.i3dm -useUriForGlb
                                           ^--- this

at the command line.

[javagl]

The PR #83 covers most of this: The functions that deal with I3DMs now usually receive an externalGlbResolver, which is a function that takes a URI and returns the Buffer with the data. (There are some degrees of freedom for things like error handling, but the main point is that external GLBs should now be supported more consistently).

The idea of generalizing the glbToI3dm command is now tracked in #84