all otData arrays are decompiled as lists (see otConverters.BaseConverter.readArray).
If one tries to call the subsetter on a GSUB table that was not read from file
but was just built from feaLib, it crases with a TypeError because it's expecting
list but finds tuples.
Traceback (most recent call last):
File /home/clupo/Github/fonttools/Lib/fontTools/subset/__init__.py, line 1462, in prune_lookups
lookup_indices = self.table.FeatureList.collect_lookups(feature_indices)
File /home/clupo/Github/fonttools/Lib/fontTools/subset/__init__.py, line 1263, in collect_lookups
if i < self.FeatureCount), [])
TypeError: can only concatenate list (not tuple) to list