Fix the positioning/anchoring of SVG text runs with bidi control characters.
The added reftests here failed both prior to and after the bidi-engine update,
although the exact nature of the failures changed due to the engine leaving the
bidi control characters in different places in the reordered text. (This is
explicitly unspecified by the Unicode Bidi Algorithm, because the control codes
are nominally removed after processing; leaving them present but invisible is
an implementation detail.)
The key fix here is that DoGlyphPositioning() anchors the first addressable
character of the text, not necessarily the absolute first codepoint, as that
may be a "deleted" control code whose position is arbitrary.
Differential Revision: https://phabricator.services.mozilla.com/D216400
bugzilla-url: https://bugzilla.mozilla.org/show_bug.cgi?id=1906978
gecko-commit: ad57df08b5385f1b5d44c77f14231c3d5108832c
gecko-reviewers: longsonr, layout-reviewers, emilio