Support full instancing of variable fonts

[papandreou]

Create full instances of variable fonts when every variation axis only has one value. Uses the hb_subset_input_pin_axis_location api that landed in Harfbuzz 6 (via papandreou/subset-font#21).

It's behind an experimental --instance flag for now, as it relies on us tracing font-variation-settings correctly and getting a bunch of stuff related to the standard variation axes right.

Makes for some nice extra savings:

$ subfont -o foo testdata/subsetFonts/variable-font-that-can-be-fully-instanced/index.html 
[...]
testdata/subsetFonts/variable-font-that-can-be-fully-instanced/index.html: 1 font (1 variant) in use, 1.65 MB total. Created subsets: 18.5 kB total
  RobotoVariable:
    normal : 10/827 codepoints used, 1.65 MB (ttf) => 18.5 kB (woff2)

$ subfont --instance -o foo testdata/subsetFonts/variable-font-that-can-be-fully-instanced/index.html 
[...]
testdata/subsetFonts/variable-font-that-can-be-fully-instanced/index.html: 1 font (1 variant) in use, 1.65 MB total. Created subsets: 1.24 kB total
  RobotoVariable:
    normal : 10/827 codepoints used, fully instanced, 1.65 MB (ttf) => 1.24 kB (woff2)

We can add support for partial instancing when the support lands in Harfbuzz.

[coveralls]

Pull Request Test Coverage Report for Build 4644256980

• 38 of 41 (92.68%) changed or added relevant lines in 2 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage decreased (-0.1%) to 89.964%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
lib/subfont.js	1	2	50.0%
lib/subsetFonts.js	37	39	94.87%

Totals
Change from base Build 4633529333:	-0.1%
Covered Lines:	1145
Relevant Lines:	1213

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💛 - Coveralls