avoid closures where possible; they lead to weird garbage collector effects!


assets
------

assets are cached. each asset type has a module (e.g. `mesh`, `image`, `sound`, etc). These have the following functions:

	* `get(filename)` - cached load of the given file. throws error if it cannot be found
	* `forget(filename` - remove the cached copy, if any of the given file
	* `clearCache()` - clear the full cache for that asset type

individual assets may have additional functions.


systems
-------

systems are pure functions (NOT closures!!). they can be embedded in tables that indicate dependencies, i.e. which other systems *must* execute before them in order for things to work correctly. They have a signature like

> `system(db, dt, [params])`

where params is a table. all parameters, including the params table, are passed in by the systemdb every frame; the system should NEVER close over them.