Detect circular dependencies among resources

[cafour]

Fixes #710.

The first commit merely satisfies my OCD. It's the second one that needs attention.

[exyi]

The first commit merely satisfies my OCD. It's the second one that needs attention.

Please don't, the same explanation as in the other PR applies - it does not really do anything for readability and pollutes history.

Anyways, the check works, but only with a the assumption that the resource dependencies are not liable, which is unfortunately not the case. I think we should keep this check you just implemented and add a second one to the point when the resources are ordered in the render phase.

[cafour]

@exyi History unpolluted.

There's something I don't understand though. How exactly can it break in the render phase if the user is not able to register resources with circular dependencies? Wouldn't the check slow down the render phase?

[exyi]

How exactly can it break in the render phase if the user is not able to register resources with circular dependencies? Wouldn't the check slow down the render phase?

The problem is that the `Dependencies` property is mutable, so I can register a resource without any dependency and create the cycle later. Unfortunatelly we certainly can't remove that setter as it would break most usages.

[tomasherceg]

I think that there would be many cases like that because many things in the configuration is mutable.
I think that we can live with that - if someone changes the configuration after startup, they deserve the exception.

[cafour]

I added a check to the point where resources are ordered and to the ResourceManager.AddRequiredResource method since both caused StackOverflowExceptions when the user tampered with the dependencies after registration.