…muizel,kvark

border-image-repeat: Round is equivalent to Repeat with the pattern size adjusted to fill the area with a whole number of repetitions. This is done by adjusting the segment's stretch_size in the shader so that it fits a whole number of times in the segment's size.

Differential Revision: https://phabricator.services.mozilla.com/D59370

[ghsync] From https://hg.mozilla.org/mozilla-central/rev/6b61867f80b123f11aa3e9252f0ac005a12787c0