Proper near plane splitting

[kvark]

The PR is built on the shoulders of #2913, #2741, servo/euclid#277, servo/euclid#291, servo/plane-split#12, and (last but not the least!) servo/plane-split#15

It uses the new clipping semantics in plane-split crate uniformly for both bounding box computation and 3d plane splitting itself, ensuring that no incorrect perspective divisions are performed (and closing quite a few TODO entries). Also adds a small reftest for the latter.

Fixes #2272

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This change is 

[gw3583]

Wow, nice! I'll review this today. Could you kick off a try-push in the meantime?

[kvark]

Launched a try in https://treeherder.mozilla.org/#/jobs?repo=try&revision=b1fc20bb9928bf6e3ed055fb9bb69c5250c8d784
Also confirmed the bugzilla case working:

[gw3583]

There are several new PASS results in the try 🎉 but there's also a few failures. Each of them seem to be the same assert:

hread 'WRRenderBackend#4' panicked at 'assertion failed: edge2.square_length() > T::approx_epsilon()', third_party\rust\plane-split\src\polygon.rs:133:9

[gw3583]

The change basically looks good to me, but I'll take another pass over it once we get that assert failure fixed :)

[kvark]

Addressed the try push issues in servo/plane-split#18
Launched a try push with those plane-split changes (overridden with a cargo patch):
https://treeherder.mozilla.org/#/jobs?repo=try&revision=f8225f77003bdf6bb87ecd0494e2dd8b779210d3&selectedJob=191881097

So far, it's a bunch of intermittents plus the expected unexpected passes.
@gw3583 please take your extra pass ;) no code is changed on this side (other than pulling the patched plane-split crate).

[gw3583]

Try run looks good to me!

[gw3583]

@bors-servo r+ 🚀

[bors-servo]

📌 Commit d331402 has been approved by gw3583

[bors-servo]

⌛ Testing commit d331402 with merge c939a61...

[bors-servo]

☀️ Test successful - status-appveyor, status-taskcluster
Approved by: gw3583
Pushing c939a61 to master...