Unload assets when leaving room

[sky-is-winning]

By keeping track of every asset the client has loaded, we're then able to unload every unneeded asset when the player leaves the current room. This massively decreases VRAM usage, but increases loading time when users re-enter a room they have already visited.

Fixes #18

[tomas-correia]

Could the unloadPrevious method possibly be moved and adapted to a more general scene class like BaseScene, so that not just room scenes are able to remove their textures (the Login and Start menus, for example)

[indexxing]

still not merged 😢