Add `System::dispose`

[torkleyy]

Requested by @Frizi.

System::dispose would be called by Dispatcher::dispose and can be used to clean up externally allocated resources.

Unresolved

• Signature: fn dispose(&self, world: &World) / fn dispose(&self, world: &mut World) / fn dispose(&mut self, world: &World) / fn dispose(&mut self, world: &mut World)
• Is it mandatory to call it? How to make sure it gets called?

[LucioFranco]

@torkleyy curious what is the difference between this and drop?

[torkleyy]

It has access to World, which is required for access to rendy::Factory in this case.

[LucioFranco]

@torkleyy that makes sense, I would imagine in that case a fn dispose(&mut self, world: &mut World) might make sense here.

[torkleyy]

Yeah, I think that makes sense.

[torkleyy]

Now, how do we make sure it gets called?

One option would be to set a flag when it gets called, and check it is set in Dispatcher::drop.