Implement variable font rendering (gvar, avar, hvar, cvar)

[Connum]

This PR implements rendering of variable fonts.

Description

• Docs (index and Glyph Inspector): Selection of predefined instances from the fvar table, all axes available as range sliders, render variations instantly. Some other improvements.
• supports gvar point transformation
• supports avar point transformation
• supports hvar advanceWidth and leftSideBearing transformation
• support for variation coordinates in the rendering options
• able to set default variation for the whole font
• works with hinting and cvar table
• adapted test render script to have less tests fail due to minor path differences

TODOs before this is ready for review:

• look into the bug that the fontkit implementation has as well
• support advanceWidth transform
• see if we can easily support CFF2 blend transforms
• see if we can easily support hvar transform
• see if it makes sense to support cvar as well
• Check toSVG results
• run unicode test suite
• try to fix more unicode tests
• Add gvar tests, including those from the unicode test suite
• Add avar tests
• Add hvar tests
• Add cvar glyph tests
• Add cvar tests
• Update API documentation in README

TODOs for future PRs:

• add cvar writing capability
• add gvar writing capability
• add HVAR writing support
• add higher level methods for working more comfortably with variation data (like implemented color glyph layers and palettes)

Motivation and Context

Finally, after supporting parsing of the required tables, we can also render glyph variations.

How Has This Been Tested?

Visual testing, comparing coordinates with other implementations, added tests.

Screenshots (if appropriate):

The tests in the unicode test suite regarding the supported tables look fine, only a few are failing due to minor rounding differences. Several unrelated tests have been fixed along the way.
passing: 212, failing: 539) as opposed to passing: 37, failing: 708 with 1.3.4.

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• I did npm run test and all tests passed green (including code styling checks).
• I have added tests to cover my changes.
• My change requires a change to the documentation.
• I have updated the README accordingly.
• I have read the CONTRIBUTING document.

[Connum]

pinging @yne @ILOVEPIE

[yne]

Impressiv work ! @Connum

Can't wait to play with this new API

Shall we squash before merging ?

[ILOVEPIE]

I'd have to do a full look through the code, but from what I can see it looks good.