…ar function. r=lsalzman

In wr/swgl/src/gl.cc, force_clear constrains skip_end to be no less than y0, but
skip_end is a horizontal position, not a vertical position, so skip_start is the
correct lower bound.

Differential Revision: https://phabricator.services.mozilla.com/D70462

[ghsync] From https://hg.mozilla.org/mozilla-central/rev/27c711af82e00ace0cb053dddf6c806b63faccd4