Implement support for KHR_gaussian_splatting and KHR_gaussian_splatting_compression_spz_2

[azrogers]

Closes #1219. This PR adds support for the new KHR_gaussian_splatting extension and the KHR_gaussian_splatting_compression_spz_2 extension. The latter is the majority of this PR - it implements SPZ decoding using the spz library, allowing glTFs with SPZ payloads to be read. It also implements backwards compatibility for the KHR_spz_gaussian_splats_compression and KHR_gaussian_splatting_compression_spz extensions, previous iterations that have already had some tilesets created using them.

[azrogers]

One random thing to note, might be a concern might not, the SPZ decoding rewrites the color channel to always be a float. This is because I was having some issues with handling conversions to the unsigned byte and unsigned short types that are also allowed, and I figured it wasn't too important to keep the exact data type. But if, for some reason, it is important - I can try to get it working.

[azrogers]

Some things I'd like to fix with the spz port (which is done here as an overlay port because there is no official spz port yet):

• Find a way to put it into a subdirectory, so the includes can be #include <spz/load-spz.h> instead of #include <load-spz.h>
• Get it to use zstd instead of zlib so we can avoid adding another dependency. This caused issues with the Unreal build for some reason last time I tried it.

[azrogers]

@kring Looks like we are held up by the 2.1.0 release of spz not yet propagating to whatever source the runners are using, but besides that I think it's good to go.