Fix crash in DL building
Fix one crash and some style changes.
This HTML crashed servo before. Key parts are `background-clip: content-box` and `direction: rtl`
```html
<!DOCTYPE html>
<html>
<style>
#span1 {
background-clip: content-box;
}
#span2
{
direction: rtl;
}
</style>
<span id="span1">Filler Text <span id="span2">txeT relliF</span></span>
</html>
```
Should I add this as a test? And where do I put this "does-it-crash?" test?
I find always passing rectangles by value a lot easier as it avoids many references and dereferences and I assume that the compiler will always use the faster one either way. If you don't like the change feel free to only merge the first commit.
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