DebugTilesPlugin: optimize performance and make it toggleable

[sguimmara]

This PR optimizes the DebugTilesPlugin by reducing its overhead, an makes it toggleable.

fix #647

Tree traversal optimizations

This PR optimizes (some) tree traversals by using a stack-based approach, rather than a recursive approach, that reduces overhead of recursion.

traverseSet() is modified from a recursive approach to a stack-based approach. toggleTiles() now uses this version.

Note: other functions in traverseFunctions.js might also benefit from using this approach, but the migration is not as easy as for toggleTiles(), as the recursion logic is entangled with the callback logic.

Disable/Enable the DebugTilesPlugin dynamically

Add the enabled property on the plugin. When toggled on, the plugin initializes its state from the current tile hierarchy. When toggled off, the existing helpers are removed, and all event listeners do nothing (except for 'dispose-model').

[sguimmara]

Before continuing further, I would like your opinion on this @gkjohnson.

The biggest performance hit of DebugTilesPlugin is the tree traversal(s) in _initExtremes(), because the traversals preprocess all tiles in the tileset, which is not necessary for the needs of _initExtremes().

In one of Giro3D examples, we have a massive tileset with a lot of nested tilesets, which means that the preprocessing happens a lot, rather than once (which would be the case with a single root tileset.json).

[sguimmara]

I actually ended up implementing the toggling itself, so it's ready for review

[gkjohnson]

Amazing! Thank you. I added a few comments but this should make the plugin a lot nicer to use.

[gkjohnson]

Why use traverseSet rather than tiles.traverse?

[gkjohnson]

Material changes are done based on the colorMode in the _onUpdateAfter function so we'll also want to make sure we revert the material changes here. This is done in the dispose function like so:

// reset all materials
this.colorMode = NONE;
this._onUpdateAfter();

Now that I'm looking at it I'm wondering if we should just call this.dispose when enabled is toggled to "false" and then call init again when it's toggled to "true". Then the plugin can clean up after itself in a consistent way, unhook any events so there's no added overhead. That may be a smaller change.

[sguimmara]

@gkjohnson I updated the PR with the following changes:

• removed the commit about toggleTiles that would use the non-recursive traversal
• simplified the toggling logic by simpling calling init() and dispose() with enabling and disabling
• added various checks (for example, dispose() does nothing if the plugin is not initialized, otherwise it would try to remove objects that do not exist and raise errors)
• added a chekbox in the main example to enable/disable the plugin:

[gkjohnson]

Thanks! Last couple comments and then this looks ready to merge. I'll have to think about how to deal with the traverseSet issue - would be nice to not have to import that utility function separately.

[gkjohnson]

Are these gates still needed here? These events will not be added to the tiles object if the flag is not enabled.

[gkjohnson]

This comment seems out of date - since we remove the events when the plugin is disabled this function will no longer be called.

[sguimmara]

@gkjohnson seems it also solves #484

[gkjohnson]

Excellent, thank you! I'll update the docs to add the field.