libcore|Bank: Added new priority level, isLoaded() method, iterate()

The ImmediatelyInCurrentThread priority causes the operation to be
performed immediately in the current thread even if the bank uses a
background thread.

One can now special importance also when unloading items. By default,
unloading occurs after any queued tasks.
Client|Model Renderer: Set up an atlas for model textures

RenderSystem now has proper glInit/glDeinit methods. ModelRenderer
sets up a texture atlas that models share for their textures.