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~ (#880)

[gkjohnson]

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