[packing] Isolate 32-bit subgraphs
[packing] Isolate 32-bit subgraphs

This implementation differs from garret's in a few ways that I will need
to ask him about. (edit: I asked him and it seems mostly okay, but I
need to ensure I'm deduplicating all nodes that have both 16 & 32-bit
inbound edges)

googlefonts/fontations,googlefonts/fontations