Shaper::save_glyph_results incorrectly starts its loop by setting glyph_span
to a length of 1.  This means that the `if glyph_span.len() == 0` test in the
inner loop will never succeed.

Instead the glyph span should start out empty, and a glyph should be added only
as the corresponding char is found.  For comparison, see the Gecko code this
was ported from:

https://hg.mozilla.org/mozilla-central/file/ab0044bf/gfx/thebes/gfxHarfBuzzShaper.cpp#l1682