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

[pcwalton]

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

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[SimonSapin]

The ligature disabling code has been manually verified, but I was unable to reftest it.

I’m not sure if this would work, but maybe you could have three files: one with "fi" (or some other ligature), one with "fi" that should be different from the first, and one with "fi" with letter spacing that should be like the latter?

… reading the spec, that might be tricky since ligatures are disabled "When the effective spacing between two characters is not zero", not just when letter-spacing is not normal. But then it’s only a "should", so maybe it’s not that important?

[pcwalton]

Yeah, I saw that language in the spec and disabled ligatures if letter-spacing was 0 as well. I'm a little wary of violating "should" in the spec just for tests, especially since we can test it another way (just with a fair amount of mucking around in FontForge).

Furthermore, it's kind of hard to find a font that Servo will actually draw ligatures with right now. This is because of #4320 — we only support the newest of the new formats for ligatures, and most fonts use the old morx stuff.

[SimonSapin]

Alright, let’s not block this PR on this, we can refine later.

[pcwalton]

r? @mbrubeck

[mbrubeck]

Disabling the GSUB table seems like a very blunt instrument. Can we disable ligatures only, by setting the "liga" feature to 0 when we call hb_shape?

[pcwalton]

I changed it to use HarfBuzz features. re-r? @mbrubeck

[jdm]

This is definitely not something that should be retried blindly. Can we please be more careful about looking at test failure logs?

[pcwalton]

r=mbrubeck

[bors-servo]

saw approval from mbrubeck
at pcwalton@07bc97e

[bors-servo]

merging pcwalton/servo/letter-spacing = 07bc97e into auto

[bors-servo]

pcwalton/servo/letter-spacing = 07bc97e merged ok, testing candidate = 914f272

[bors-servo]

all tests pass:
success: http://build.servo.org/builders/linux1/builds/276
success: http://build.servo.org/builders/linux2/builds/275
success: http://build.servo.org/builders/android/builds/343
success: http://build.servo.org/builders/mac1/builds/492
success: http://build.servo.org/builders/mac2/builds/493

[bors-servo]

fast-forwarding master to auto = 914f272