Before this change, feaLib had assigned a lookup index at the same
time as compiling each lookup. For chains, the implicit assumption was
that the chain's targets would always come before the contexual chain.
Normally this was indeed the case, but feaLib (and also makeotf)
sometimes merge several chain targets into one single lookup,
and then this assumption was not true anymore.
In the new version, the lookups get compiled in a separate pass,
after assigning lookup indices.
https://github.com/behdad/fonttools/issues/463
https://github.com/behdad/fonttools/issues/445
Not sure whether it makes much sense to define a contextual chain
that points to a GSUB type 3, but the OpenType feature file syntax
does not explicitly forbid it. Adobe's `makeotf` tool rejects this
kind of input with "Contextual alternate rule not yet supported";
this implies that the construct is valid (albeit definitely exotic).
Before this change, the compiler had (essentially) implemented lookup
references by inlining the statements of the invoked lookup into the
current feature block. After this change, the lookup gets compiled
separately, and any call sites make explicit calls.
