In PDF one can output a line using a delta offset between different glyphs;
currently the sequence we use to out put three consecutive glyphs not directly
adjacent is

xxx 0 Td /F1 Tf  <01> Tj
yyy 0 Td <02> Tj
zzz 0 Td <03 > Tj

If i knew the original glyph widths i could use the more comprehensive form
xxx 0 Td /f1 Tf [ <01> yy <02> zz <03> ] TJ
where the numbers are delta values in width from the glyph width (the feature
originally was invented for kerning).

Text documents exported to PDF would be smaller this way; for this to work the
SalLayout would need to output the advance widths (as it currently does) as well
as the original glyph advance widths.