Ufo2ft

- [featureWriters] Added `quantization` option to `MarkFeatureWriter` and `KernFeatureWriter`, to round values to the nearest multiple of a given integer value, thus potentially optimizing the GPOS size (513, 516, 517).
- [MarkFeatureWriter] Ensure `mark` is always written before `mkmk` feature (506, 512).
- Bump minimum required fonttools >= 4.25.1. Fixes a bug with the `TTGlyphPen` when compiling glyphs with open contours (googlefonts/nanoemoji304).

- Added support for Unicode Variation Sequences via lib key `public.unicodeVariationSequences` (495, 496, 162).
- outlineCompiler now uses the new fonttools' TTGlyphPointPen to avoid modification of the order of TrueType contour points (503).
- Added support for `meta` table from "public.openTypeMeta" UFO lib key (509).
- Fixed issue whereby MarkFeatureWriter's markClasses were inserted after manual `mkmk` feature, leading to compilation error (504, 505).

* While compiling variable fonts, only post-process the final merged font. This can save ca. 14-24% of the compilation time, depending on the sources. (486)

- Use `cffsubr` by default for both CFF 1.0 and CFF2 (465). Make `compreffor` an optional requirement (464).
- Added support for ` Automatic Code` special marker comments to allow inserting auto-generated features in specific places of the features.fea, similar to Glyphs.app (458, 482, 351).
- Allow to run ufo2ft filters and featureWriters from the command line, which can be helpful for testing purposes (470, 473).
- Added `loadFilterFromString` function, pass `filters` argument to compile functions and PreProcessor (466, 479, googlefonts/fontmake737).
- Added GDEF feature writer (enabled by default) that generates GlyphClassDefs based on UFO public.openTypeCategories, and also generates ligature caret statements based on "caret_" or "vcaret_" named glyph anchors (480).

- [flattenCompontents] Fixed issue with mixed contour+component glyphs in nested components (448).
- [flattenCompontents] Fixed issue with transformed nested components ending up with the wrong translation offset (450).