Viewer: add support for hdr environments

[ryantrem]

When loading an environment from a .hdr file, we need to use HDRCubeMap rather than CubeMap. To enable this:

1. Add a helper function for getting the file extension from a url.
2. Viewer tries to determine the extension from the environment url.
3. If the extension is .hdr, we use HDRCubeMap instead of CubeMap.
4. In either case, the necessary cube map texture module is dynamically imported.

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