Use pointers instead of references, in preparation for upcoming change.

We are not using it for anything lse it seems.

Nothing changed...

Eventually this will become part of a yet-to-be-written hb-ot cmdline
tool.

Was causing assertion failure in shape_plan().

The old code doesn't work with all fonts, as Khaled has reported.

Commit 4c4e8f0e broke contextual lookups
by making the recurse() function always return false.

Reported by Khaled.  Test case: لا in Amiri.

Untested.

All of collect_glyphs() complete and untested now.