don't drop implied oncurves for interpolatable TTFs when building a VF

[anthrotype]

this should only be done at the end in the final VF, otherwise masters may end up with incompatible glyphs...

[anthrotype]

Added a test with a reproducer, which fails as expected.

https://github.com/googlefonts/ufo2ft/actions/runs/7804773235/job/21287495551?pr=817#step:5:217

WARNING  fontTools.varLib:__init__.py:1069 Failed to drop implied oncurves for 'o': Incompatible flags for glyph at master index 1: expected array('B', [1, 0, 0, 0, 0, 0, 0, 0, 0, 1, 0, 0, 0, 0, 0, 0, 1, 0, 0, 0, 0, 0, 0, 0, 0, 1, 0, 0, 0, 0, 0, 0]), found array('B', [1, 0, 0, 0, 0, 0, 0, 0, 0, 1, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0])
INFO     fontTools.varLib:__init__.py:1170 Dropped 0 on-curve points from simple glyphs in the 'glyf' table
[...]
WARNING  fontTools.varLib:__init__.py:335 glyph o has incompatible masters; skipping
``

[behdad]

Does varLib do it?

[anthrotype]

this issue might have been noticed earlier if varLib errored instead of just warning: fonttools/fonttools#2572

[anthrotype]

Does varLib do it?

yes, we do it either in the TTGlyphPen (for the static fonts) or in varLib (for the variable ones)

[anthrotype]

this is a regression from #801 when we moved things around. The previous code was pop'ing the the dropImpliedOnCurves parameter from the kwargs before making the outline compiler for the VF's interpolatable masters and then passing it on to varLib.build

see

ufo2ft/Lib/ufo2ft/__init__.py

Line 645 in 85e28b7

dropImpliedOnCurves = kwargs.pop("dropImpliedOnCurves")