v0.3.0: Progressive loading

Support progressive loading for .ksplat and .splat files. Support for streaming .ply files will be included in a future release.

• Enabled via the streamView option to addSplatScene. It is false by default to preserve existing behavior.
• Currently streaming is only supported with the loading of a single scene; streaming of multiple scenes will be added in a future release.
• Enable Viewer.dispose() to be called while scene loading is in progress

Beaking Changes:

• Updated .ksplat format so that it can be streamed, and to allow for better compression, multiple scene sections, and an optimized layout that yields better sorting performance. Older versions of .ksplat files will no longer be supported.
• showLoadingSpinner parameter for Viewer constructor is now showLoadingUI

Thanks to @gotoeasy for helping me test this feature out and find bugs!