Add support for font-variant

[jdahlin]

This PR adds support for:

• font-variant-alternates: normal, historical-forms.
• font-variant-caps: normal, small-caps, all-small-caps, petite-caps, all-petite-caps, unicase, titling-caps.
• font-variant-east-asian: normal, jis78, jis83, jis90, jis04, simplified, traditional, proportional-width, full-width.
• font-variant-emoji: normal, text, emoji, unicode.
• font-variant-ligatures: normal, none, common-ligatures, no-common-ligatures, discretionary-ligatures, no-discretionary-ligatures, historical-ligatures, no-historical-ligatures, contextual, no-contextual.
• font-variant-numeric: normal, ordinal, slashed-zero, lining-nums, oldstyle-nums, proportional-nums, tabular-nums, diagonal-fractions, stacked-fractions.
• font-variant-positional: normal, sub, super.
• font-variant shorthand: see above

The CSS properties are parsed, styled and passed on to layout which ultimately passes them on to harfbuzz which shapes them based OpenType Tags, using
dlig, clig, calt etc

This PR includes font-feature-settings honoring feature precedence

• Note: font-variant-alternates lacks support for most css functions such as stylistic(feature-name)
• Note: font-variant-east-asian requires better script shaping support to be actually useful.
• Note: font-variant-emoji is only parsed/styled, layout/rendering is a todo.

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[jdahlin]

wpt css/css-fonts before:

Ran 496 tests finished in 169.8 seconds.
• 206 ran as expected. 0 tests skipped.
• 13 tests crashed unexpectedly
• 172 tests failed unexpectedly
• 14 tests timed out unexpectedly
• 94 tests had unexpected subtest results

wpt css/css-fonts after:

Ran 496 tests finished in 184.2 seconds.
• 336 ran as expected. 0 tests skipped.
• 13 tests crashed unexpectedly
• 42 tests failed unexpectedly
• 14 tests timed out unexpectedly
• 94 tests had unexpected subtest results

This fixes at least 130 tests.

A few of the WPT tests have been imported into the testsuite that had to be tweaked to allow custom fonts to be loaded.

[AtkinsSJ]

Hi, thanks for making a start on font features! I've mostly stuck to looking at the CSS code as that's my area.

Your final commit looks like it's mostly moving code that was added earlier in the PR. Please apply those changes to the earlier commits instead, it makes the git history much easier to follow.

[jdahlin]

Hi, thanks for making a start on font features! I've mostly stuck to looking at the CSS code as that's my area.

Your final commit looks like it's mostly moving code that was added earlier in the PR. Please apply those changes to the earlier commits instead, it makes the git history much easier to follow.

Thanks for the review! I had to do significant refactoring to support writing to the shorthand property, that why the last commit looks weird. I'll recreate the different commits to avoid shuffling things around unnecessarily and maintain the git history clean.

[jdahlin]

@AtkinsSJ Thanks for the thorough review, it's been really helpful and is improving the PR significantly. I'm still learning quite a bit of C++ while doing this. Two conversations are unresolved as I could not figure out how to make them work the way you suggested. Everything should be solved now!

[AtkinsSJ]

Forgot to mention these before! Otherwise I agree with Jelle, overall this is in decent shape and can be merged.