In the long term, we might want to make a different low-level API
for building ChainContextPos lookups; for now, this should fix the
current bug with SequenceIndex.
Resolves https://github.com/behdad/fonttools/issues/517.
There should be no semantic difference from this change,
since dependent lookups (the chain targets) are never directly
invoked by a feature. But the output of feaLib becomes more
similar to the output of makeotf, which helps debugging.
Before this change, we had only emitted a SinglePos (GPOS type 1) lookup
for a statement like `pos A' B' 20`; after this change, we always emit a
chain rule even if there is no context. There is a semantic difference if
the rule is preceded by a `ignore pos` statement. Omitting context-free
contextual chains was actually not a (premature) optimization, but an
artifact that came from the representation of glyph patterns.
https://github.com/behdad/fonttools/issues/516
