Gracefully avoid generating empty border segments

[kvark]

Fixes https://bugzilla.mozilla.org/show_bug.cgi?id=1489644
Also related to #2715 (comment)

The change makes us avoid doing meaningless work and then stumbling upon unexpected data. Good for both performance, stability, and the health of the users ;)

---

This change is 

[kvark]

Try push has a few failures - https://treeherder.mozilla.org/#/jobs?repo=try&revision=20051aa2e74a650b578975000da243b5609813d3&selectedJob=198925969
Although this is with replacing the whole gfx/webrender as opposed to patching the working version.

[gw3583]

Generally looks good - although at least some of those try failures do look to be border related, so I'll take a closer look once those are resolved.

[kvark]

Another try push that is carefully rebased on the current WR revision of Gecko: https://treeherder.mozilla.org/#/jobs?repo=try&revision=8c0469da6856e6d611d0fb15a4c112ad93ef1387

[gw3583]

Thanks! Looks like the failures are probably from this patch, I think. https://hg.mozilla.org/mozilla-central/raw-file/tip/layout/tools/reftest/reftest-analyzer.xhtml#logurl=https://queue.taskcluster.net/v1/task/BCz2XciXTQi6PP1pUaUlNQ/runs/0/artifacts/public/logs/live_backing.log&only_show_unexpected=1 has a set of tests that look border related.

[bors-servo]

☔ The latest upstream changes (presumably #3054) made this pull request unmergeable. Please resolve the merge conflicts.

[emilio]

Closing for the same reason as #3056.