Ufo2ft

- [featureWriters] Ensure CursFeatureWriter comes first, before kerning mark feature writers; in some non-HarfBuzz implementations the GPOS lookup order matters (864).
- [kernFeatureWriter] Ignore zero-valued class-class kerning pairs when generating variable kern feature as they are no-op (866).
- Take discrete axes into account when checking for feature compatibility across masters (867).

- cursFeatureWriter: Remove stray print()

- kernFeatureWriter: Support sparse masters (859)
- cursFeatureWriter: Support multiple entry/exit anchor pairs (863)

- Removed unnecessary assertions that may sometime trigger an AssertionError while setting a glyph's right margin in dottedCircle filter (849).

- Fix variable anchors when DesignSpace source specifies a layer (842).
- Update the old kernFeatureWriter2.py to be able to output variable FEA for use when compiling features once per VF (841, 845).

[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).