Don't attempt to trim letter-spacing at start of line in textframes being used for SVG text.
SVG text elements in general don't have a clearly-defined concept of "lines", so we can't
know where it would be appropriate to trim.
Differential Revision: https://phabricator.services.mozilla.com/D216506
bugzilla-url: https://bugzilla.mozilla.org/show_bug.cgi?id=1907772
gecko-commit: e75b0f8b9b420a182bb59d05df6c47520d57e579
gecko-reviewers: longsonr