Follows #5442.
- text.setGlyphAlternate keeps the default glyph as a choice when a
feature of the character's style (salt, a stylistic set) draws another
glyph for it, so the default glyph is drawn. A character that is not
laid out (overflowing text) is shaped alone in its style to decide.
gid: null reports the glyph the style draws.
- The Glyphs panel's alternates grid shows the default glyph first when
the font does not list it, so a chosen alternate can be put back.
- A test for a chosen character that starts a longer grapheme (f with a
combining acute), which is not split from its mark.
- The alternates tests in the engine hold the shared font lock while
they add fonts to the global font database.
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