Fontmake

- ufoLib with lxml support is now a hard requirement of fontmake. Lxml provides precompiled wheels for all python versions and platforms that fontmake supports so this is ok.
- Added self-contained python zipapps for all the platforms/python versions that are currently supported by fontmake. Just download the `fontmake*.pyz` file that matches your python, platform and architecture and run it like a normal executable. No need to create virtual environments or install any dependencies. They are all embedded in the .pyz file (which is actually a zip file with some magic bootstrapping python code and the whole site-packages embedded). Please report any issues that you find.

- Added new option `--overlaps-backend` that allows to optionally specify "pathops" instead of the default "booleanOperations" as the backend for the ufo2ft RemoveOverlapsFilter.
Install fontmake as `fontmake[pathops]` to enable support for this, or install skia-pathops separately if you prefer.
- Added new option `--validate-ufo` to enable ufoLib validation upon reading and writing UFOs. It is disabled by default for marginally better speed.
- You can install as `fontmake[lxml]` (or install lxml yourself) to enable faster UFO reading/writing using the latest ufoLib.
- NOTE: You can specify multiple extras at the same time within the square brackets, e.g. `pip install --upgrade fontmake[pathops,lxml]`
- Updated minimum installation requirements:

* glyphsLib >= 3.0.3
* ufo2ft >= 2.2.0
* defcon >= 0.5.2

- glyphsLib >= 5.1.6

Fixed a regression whereby `--keep-overlaps`, `--keep-direction` and `--conversion-error` options were being ignored instead of passed on to the ufo2ft compiler (7eb44b48).

- Support new lib key for the "Keep Glyphs" custom parameter introduced since glyphsLib 2.3 (446).
- Don't generate default kern/mark/mkmk feaures when `--interpolate-layout` option is passed, as they will be overwritten by the interpolated GPOS.
- Bumped minimum requirements:
* fonttools >= 3.28.0
+ Fixed an issue in varLib's model resolution algorithm
+ It now uses a new rounding function better suited for visual data such as coordinates, advances, deltas, etc.
* ufo2ft >= 2.1.0
+ Fixed issue when hhea/vhea metrics contain float values which need to be rounded to integers
* glyphsLib >= 2.4.0
+ Fixed regression in the generated `designspace.filename` when a custom `--family-name` is provided that could lead to fontmake outputs overwriting each other
+ Added support for "Reencode Glyphs" instance custom parameter.

Please consult the respective changelogs for the full list of changes.

- Correctly handle "Keep Glyphs" custom parameter when subsetting the font: the global list always has priority over individual glyph export flags (381).
- Fixed incorrect subsetting when the UFO glyphOrder and the built TTFont objects's glyph order don't align (e.g. ".notdef" glyph not in the first position) (443, 444).
- In `FDKFeatureCompiler`, check makeotf exit code to determine whether build was successful.