ImageOverlayPlugin: Add separate uv channels per overlay.

[gkjohnson]

Related to #1155

TODO

• Disposal of texture is still causing issues - some times "get" is returning "null". (zoom in on google maps demo)
  • this is occurring because the tile is being deleted before the tiles can all be drawn / used
  • we can add an abort signal or id to early-out in some of the functions
• Reduce some redundancy of overlay update functions.
• Use scheduled texture unload