Load new scene file from disc or web

Load a new scene from disc, the web, or Emscripten's virtual file system. Optionally save the previous scene. On the C++ side, make sure all resources associate with previous scene are freed.

Start by locating the existing scene-loading entry point and resource ownership code, then trace how file input currently works. Done means scenes can load from disc, the web, and Emscripten’s virtual file system, optionally preserve the previous scene, and free its associated resources.

Written by the indexing model from the issue text.