Kern splitter: Different number of kern features per master

In one of our fonts, with ufo2ft 2.29.0b1 I get the following message when compiling a VF:

```
Couldn't merge the fonts, because some values were different, but should have
been the same. This happened while performing the following operation:
GPOS.table.FeatureList.FeatureCount

The problem is likely to be in Something Cd Hair Italic:
Expected to see .FeatureCount==11, instead saw 9

Incompatible features between masters.
Expected: kern, kern, kern, kern, kern, kern, kern, kern, kern, mark, mkmk.
Got: kern, kern, kern, kern, kern, kern, kern, mark, mkmk.
```

Will dig deeper later.

Start by reproducing the variable-font compilation with ufo2ft 2.29.0b1 and compare the GPOS FeatureList values across masters, especially the differing kern feature counts. Done means the reported incompatible features no longer prevent compilation, with the behavior verified against the affected font.

Written by the indexing model from the issue text.