CFF2 Variable font with unsusual UPM vertical metrics different/broken in Mac OS

Hey,

I've created a [test case](https://www.dropbox.com/s/zhs7z5u0ix7y5pp/Testfont-VF-CFF2.zip?dl=0) for a .glyphs source with two masters and a single letter `H`. The UPM is **3000**.

Fonts have been generated with fontmake:
```
fontmake -g Testfont.glyphs -o variable --output-path Testfont.ttf
fontmake -g Testfont.glyphs -o variable-cff2 --output-path Testfont.otf
```

Using fontmake to extract UFOs and reviewing them, the sources seem extracted correctly and _match_ the glyphs file. Inspecting the generated font's `OS/2` and `hhea` tables they appear to have _identical_ values.

Yet, the generated OTF and TTF files differ significantly in how they are rendered. Mac OS does not seem to deal well with the generated OTF files, e.g. OS font preview:

TTF selected in Finder:

OTF selected (note the 1 pixel speck where the H in the font is located — for an actual production font with UPM 2100 and a full alphabet the entire font is previewed as little one-pixel specks — if this is a cropped cut off or if the entire glyph is scaled to be displayed as 1px I cannot tell):

Comparison for an actual font:
TTF variable font preview:

OFT variable font preview:

The fonts looks similar in most programs and their cut off and vertical metrics behave like I'd expect, e.g. Adobe CC, or see below a screenshot of both files in Fontgoggles:

However, for example in Drawbot (which I believe uses the CoreText python bindings) the files appear equally cut off/cropped as in the Mac OS preview. Switching the font UPM to something more conventional like 1000 there no longer is this grave difference in _rendered_ the vertical metrics/cutoff.

Happy to test more to help narrow this down.

Start with the linked Testfont.glyphs case and reproduce the generated variable-cff2 and variable TTF outputs using the shown fontmake commands. Compare their OS/2 and hhea tables, then check rendering in macOS Preview and Drawbot; the issue is done when the cause of the different vertical metrics is identified and the OTF renders consistently.

Written by the indexing model from the issue text.