Ufo2ft

Fixed regression in compileVariableTTF rejecting valid keyword arguments (551, googlefonts/fontmake817)

- Drop Python 3.6 support, require Python >= 3.7 (541)
- [COLR] Don't copy color layer refers to a glyph in the same UFO layer,
don’t make an pessary copy of the glyph (538)
- [preProcessoer] Apply `ExplodeColorLayerGlyphsFilter` to interpolatable
TTFs (537)
- [featureWriters] Add writer for `curs` feature (476)
- [doc] Document color layers lib keys (536)

- [filters] Also transform advance width and height when applying a scale through
the Transformations filter (490, 515).
- [COLR] Define new 'colrClipBoxes' lib key, to be used with ``colorLib.buildCOLR``
for building COLRv1 ``ClipList`` table (521).

- [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)