Optimization: draw solid, single color borders as rect + clip.

[glennw]

Due to various recent optimizations and features added for the
segment logic, it's now feasible to start drawing borders as
a solid rect + clip mask.

For now, we start with only the simplest and most common case,
borders with style: solid and a single color.

This optimization applies in a couple of ways:

• Fewer shader switches and better batching. Specifically, we
  can often batch these simple borders into an existing solid
  rect batch. Additionally, the edge segments of borders are
  typically considered opaque segments, which are more efficient
  to batch, and help with early z-reject of other fragments.

• Once we start to cache clip masks between elements, and also
  across frames / display lists, we'll automatically get some
  wins here by only rendering a border corner mask once where
  applicable. Many real world sites use a common border radius
  on all buttons, for example, so this can be a significant
  optimization in these cases.

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This change is 

[glennw]

r? @kvark

This includes the 2 commits from #2618, it might be easier to review this after that's landed - only the last commit here is new.

Try run is here:

https://treeherder.mozilla.org/#/jobs?repo=try&revision=2d090150fea8692f35a4cbc961076d9a801f8930&selectedJob=172218279

Same results as in #2618 plus 1 new PASS in addition to those.

[glennw]

Rebased on master so there is just the one commit now.

[kvark]

Once we start to cache clip masks between elements

I have a strong dejavu feeling about this :)

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Reviewed 11 of 11 files at r1.
Review status: all files reviewed at latest revision, all discussions resolved.

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Comments from Reviewable

[kvark]

@bors-servo r+

[bors-servo]

📌 Commit d26c75c has been approved by kvark

[bors-servo]

⌛ Testing commit d26c75c with merge df73569...

[bors-servo]

☀️ Test successful - status-appveyor, status-taskcluster
Approved by: kvark
Pushing df73569 to master...