gfx: Implement `letter-spacing` per CSS 2.1 § 16.4.

The ligature disabling code has been manually verified, but I was unable
to reftest it. (The only way I could think of would be to create an
Ahem-like font with a ligature table, but that would be an awful lot of
work.)

Near as I can tell, the method used to apply the spacing (manually
inserting extra advance post-shaping) matches Gecko.

r? @SimonSapin 
Disabling the GSUB table seems like a very blunt instrument.  Can't we disable ligatures only, by setting the `"liga"` feature to `0` when we call `hb_shape`?