[OT] Get list of lookup subtables once and loop through them
This speeds up shaping the Amiri font by over 15%.
This was primarily needed for my work on OpenType GX, since
we will be collecting only sublookups that are "active" for
current font instance; but it's a nice boost in general as
well.
We might, in the future, collect subtables in the lookup_accel.
That would also allow us to do a per-subtbale set-digest, which
should speed things up some more, specially for ContextChainFormat3
lookups... Amiri, for example, contains one lookup with 53
subtables!