Fontmake

- Update minimum requirements:

* fonttools >= 4.2.5

Bumped minimum fonttools (4.2.3) and glyphsLib (5.1.1) requirements.

Fixed error when building from Glyphs file and one glyph contains duplicate layers with the same name (googlefonts/glyphsLib566).

Fixed bug when building variable fonts and GPOS MarkBasePos subtables need to be split because of offset overflows: the mark class values were not correctly updated leading to invalid mark-base attachments (fonttools/fonttools1797, googlefonts/noto-source145).

Fixed computation of bounding boxes when sources or interpolated instances use floating-point numbers for coordinates and component offsets (593).

Bumped requirements:
* fonttools 4.2.0
* ufo2ft 2.11.0
* fontMath 0.5.2

Added ``--debug-feature-file`` option to dump content of OpenType features to a file, to aid debugging auto-generated features (kern, mark, mkmk, etc.).

Bumped requirements:
* ufo2ft >= 2.10.0
* ufoLib2 >= 0.5.1

- Use ufoLib2 instead of defcon to load UFOs (599).
- MutatorMath is now optional. Only required if you use `--use-mutatormath` option.
- Bumped minimum requirements:
* fonttools >= 4.1.0
* glyphsLib >= 5.1.0
* ufo2ft >= 2.9.2
* booleanOperations >= 0.9.0
* ufoLib2 >= 0.5.0

- Ignore glyphs in 'public.skipExportGlyphs' when subsetting for 'Keep Glyphs' custom parameter. Fixes AssertionError when both 'public.skipExportGlyphs' and 'Keep Glyphs' are present in UFO (591).
- Log the name of the instance UFO currently being generated.