Slack channel-name characters were clipped at fractional device scales. Render
text runs at their exact device positions and keep a device-space text run's
clips unsnapped, so a fractional clip edge no longer rounds inward and shaves
the last glyph; a snapping primitive instead snaps every clip in its chain to
the device grid. Also rounds out the bounding rect of non-snapped primitives and
drops the now-dead is_offset_only reference-frame flag.
Updates the affected wrench reftest baselines and adds a unit test asserting the
device-text clip snap policy (the rendered difference is a sub-pixel clip shift
that headless software rasterization cannot surface, so it can't be reftested).
Differential Revision: https://phabricator.services.mozilla.com/D310543
Glenn Watson
98b6b93145