BatchedTilesPlugin: Improvements

[gkjohnson]

Related to #800

• Support multiple batched meshes / array textures to work around array texture depth limit.
• Abstract management of meshes, textures into a separate class
• Allow for scene objects to be removed from the cache object entirely rather than replacing it with an empty group
• Add dedicated demo w/ toggle
• Consider variant that doesn't delete geometry, only adds visible tiles to the batched mesh (UnloadTilesPlugin: Add time delay and bytes target #880)

[gkjohnson]

The recent addition of the UnloadTilesPlugin and "discardOriginalContents" flag allow us to always fit in the texture limit.