Ufo2ft

[markFeatureWriter] When building variable anchors from designspace sources, don't crash if a non-default, 'sparse' UFO source does not contain some glyph; simply skip it (840).

- Make it easier to use variable features by allowing to put them in the default source only (833).
- [instructionCompiler] When compiling TrueType glyph instructions, round TTGlyph's component transforms with floatToFixedToFloat(precisionBits=14) before computing their hash; the hash stored in the UFO glyph lib, which the computed hash is compared against, was itself computed from compiled (i.e. rounded) values (814).

- [BaseIFilter] Fixed performance regression when decomposing composite glyphs in fonts with large number of masters and/or nested components.
We now use a cache to avoid recursing too much when gathering locations from component glyphs (829).

- Some bug fixes and better error handling while building MATH table (820).
- Fixed a error while building variable GDEF ligature carets (821).
- Added new ``ufo2ft.instantiator`` module, copied over from ``fontmake.instantiator`` and adapted to also work on defcon Font objects, in addition to ufoLib2.Font (825).
- Support compiling variable TTF or CFF2 fonts with 'sparse' composite glyphs which define either more or fewer sources than the glyphs used as components. We use the instantiator to interpolate glyphs on the fly while decomposing, or we add empty placeholders in TTF masters for missing components in the non-default masters so the sparse composites can be built with varLib (googlefonts/glyphsLib954, 826).

TODO