…ender_task.rs:37 r=gw

Adding a repro-case reftest that asks for a 19996x5000 RenderTask (at
-p1), then fix it in analogy with the clamping to reasonable values that
happens for `NormalBorder`.

Differential Revision: https://phabricator.services.mozilla.com/D65660

[ghsync] From https://hg.mozilla.org/mozilla-central/rev/14caf2826f00fc609f7fe55d8df14e7e08fd1ced