Compatibility checker should hint at DS5 new feature of discrete axes

In DS5, we can build VFs from a subset of masters. The compatibility checker runs first but it checks that *all* masters are compatible, not just the subset that are being merged.

i.e. I am still getting this error:

```
ERROR:fontmake.compatibility:Fonts had differing anchors in glyph I:
ERROR:fontmake.compatibility: * MyFont Text Regular, MyFont Text Bold, MyFont Regular, MyFont Bold had "bottom, ogonek, top"
ERROR:fontmake.compatibility: * MyFont Text Regular Italic, MyFont Text Bold Italic, MyFont Italic, MyFont Bold Italic had "bottom, ogonek, top, topleft"
```

even though I am building the Italic and Regular VFs separately.