Support complex clipping regions with multiple nested rounded corner clipping regions

[pcwalton]

If you have the intersection of multiple nested border corners, we don't render that right. We need to.

[kvark]

I believe @glennw proposal to #498 will address this (pasting the relevant text from email here):

My idea is to remove that viewport_rect code, and introduce a new struct, which is a ClipRegionStack or something similar. As we traverse the tree in cull_layers() we would push the mask rect / transform into that as needed. This could return some kind of identifier for the clip region that applies to this part of the tree (perhaps store in the Layer structure?).

[jrmuizel]

Here's an example: https://people-mozilla.org/~jmuizelaar/servo/border.html

[glennw]

@kvark Yes, the plan for the clip region stack we've discussed should cover this case too.

[glennw]

This happened. There are still some bugs and edge cases, but we can open specific issues for those as we encounter them.