Convert code to Plugins

[gkjohnson]

Related to #529, #525, #490, #185

• IndexedDB Tile Cache plugin (#632)
  • Create plugin that writes fetch buffers to indexedDB or Cache so they can be quickly read from disk rather than waiting for data to fetch. Though this is possibly just bypassing the browser cache it's guaranteed to work even when the asset headers aren't set correctly.
  • Could reset on page load / unload or retain data between reloads. Possible to set a "time to live" on the cached data?
  • Other
    • Could increase queue size on initial load.
    • See skipping settings for cesium's tiles - current issues with the threshold setting means that when the the camera zooms out and parent tiles must load then the tiles "pop" to the parent LoDs.
• BatchedMesh plugin (#628)
  • Create plugin that packs tiles data into BatchedMesh to reduce draw calls (see this demo).
• Data preupload? (#632)
  • Create plugin to pre upload geometry, textures before allowing geometry to render to reduce frame stalls. May not have a large impact, though?
• Google / ion authentication (#629, #630)
  • Move ion and google cloud authentication handling to a plugin
• Debug visualization (#631)
  • All debug visualizations could be moved to a plugin to avoid having to wrap the base class and can be toggled more easily
• Metadata system Using the manager is good enough for now, I think
• Draco, meshopt etc support

[gkjohnson]

Closing in favor of #628, #629, #630, #631, #632