Render pass with no attachments causes subtraction underflow.

[sadmac7000]

https://github.com/amethyst/rendy/blob/master/graph/src/node/render/pass.rs#L718

The subtraction here underflows when attachments.len() == 0. This causes a crash in developer mode.

[zakarumych]

This use-case is not supported by RenderPassNodeBuilder because it must figure-out framebuffer dimensions. I guess an error would be more appropriate then underflow.