Auto merge of #248 - nical:empty-size, r=kvark
Auto merge of #248 - nical:empty-size, r=kvark

Add Size::is_empty_or_negative.

There are a places in webrender where we go through the boilerplate of checking width and height separately, it'd be more convenient to have something handy.

I'm tempted to call it `is_empty` since in practice negative sizes tend to be treated as empty in webrender.

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servo/rust-geom