load tiles from network if they aren't in the local asset directory

[oli-obk]

not pretty, but it works

cc #18
cc #5 (does not fix this yet, as we aren't caching downloaded assets locally yet)

this is very hacky and requires you to adjust your asset folder. You need to move everything from the models subfolder into the main assets folder. How this came to be:

1. loading from the server uses some/path/x/y.glb.gz while loading from disk uses some/path/x_y.glb
2. bevy tries to load textures and any other things required by a glb relative to the directory of the glb
3. bevy_http uses the directory of the server for loading the glb but then sub-assets are loaded from the default path. This is probably a bevy deficiency around custom asset loaders.
4. I created a fork of bevy_http that uses a placeholder string instead of a slash (but converts that placeholder to a slash before doing the network operation)
5. but now (due to 3.) it looks for textures in assets/textures. So I moved everything up instead of trying to figure this out correctly for now. There are FIXMEs in the code so we don't forget.

[oli-obk]

This change from Scene to Gltf is necessary because I was unable to implement gz decompression for arbitrary handles. We'll be able to switch back to Scene in the future