Adjust the three.js tile parsing function, parsing load index, and preprocessing content to be managed by the base tiles renderer

[gkjohnson]

ie - remove the need for the three.js tiles renderer to modify the tile and cache objects directly. Instead these functions should return new objects with cached data needed for later use.

For example

• parseTile should return an object with the scene and cached material, geometry, etc information instead of modifying "tile.cached" in place. This would let the parent object manage the load index etc
• preprocessNode should do the same - ie not modify the tile object to add a "cached" object but instead return an object that the base can manage.

[gkjohnson]

Could make implementation like the fade implementation (#421, #297) which needs to defer the full disposal of tile contents and ends up disposing of tile contents, reuploading them to the gpu, and then disposing again.

[gkjohnson]

Alternatively the abort signal could be passed into the "parseTile" function to indicate whether the tile parsing should be "cancelled" or not so a separate "loadIndex" does not need to be tracked. Likewise the same could likely be done in the "TilesRendererBase" function (checking for the signal.aborted rather than tracking an index) (can also remove the error callback definition and inline it).