[WIP] Round up the stretch size

[kvark]

Fixes https://bugzilla.mozilla.org/show_bug.cgi?id=1401665
Pending try: https://treeherder.mozilla.org/#/jobs?repo=try&revision=e138e9e5aa30bd5f1c40501c990a73dc056a60fe

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This change is 

[kvark]

There is a failure in "file:///builds/worker/workspace/build/tests/reftest/tests/layout/reftests/backgrounds/background-tiling-zoom-1.html", I'm looking into it.

[Darkspirit]

Debian Testing, KDE, Radeon RX480, 2560x1440
mozregression --repo try --launch e138e9e5aa30bd5f1c40501c990a73dc056a60fe --pref gfx.webrender.all:true startup.homepage_welcome_url:"https://bugzilla.mozilla.org/attachment.cgi?id=8910415|https://bugzilla.mozilla.org/attachment.cgi?id=8910430|https://bugzilla.mozilla.org/attachment.cgi?id=8916296|https://dev.terrax.net/"

One thin line is left.
Second column, try build: WR is sharper/coarser as non-WR at the place where the line was.
(I promise to replace this ugly PNG loading circle with an SVG.)

[kvark]

@Darkspirit wow, I don't know what to say... incredible analysis, thank you!

[bors-servo]

☔ The latest upstream changes (presumably #2619) made this pull request unmergeable. Please resolve the merge conflicts.

[kvark]

To be resumed later