Add support for instancing variable fonts (pinning variation axes)

[papandreou]

Use the hb_subset_input_pin_axis_location function from Harfbuzz 6.0.0 to instance variable fonts.

Prerequisite: harfbuzz/harfbuzzjs#76

[coveralls]

Pull Request Test Coverage Report for Build 4631014735

Warning: This coverage report may be inaccurate.

This pull request's base commit is no longer the HEAD commit of its target branch. This means it includes changes from outside the original pull request, including, potentially, unrelated coverage changes.

• For more information on this, see Tracking coverage changes with pull request builds.
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• For a quick fix, rebase this PR at GitHub. Your next report should be accurate.

Details

• 5 of 5 (100.0%) changed or added relevant lines in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+1.2%) to 88.889%

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Totals
Change from base Build 4631003556:	1.2%
Covered Lines:	52
Relevant Lines:	58

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💛 - Coveralls