[subset] Add missing face reference in hb-subset-plan plus ensure all struct members are cleaned up on destroy.

New API:
- hb_set_previous()
- hb_set_previous_range()

New API:
- hb_blob_copy_writable_or_fail()

To signify that unlike rest of library, returns nullptr on failure.

Makes more sense given what this prints, err, dumps.

So it's easier to cross-reference from output of print-indic-table.

Was writing source font's offsets. Ouch.

To fix this warning:

cc1: warning: command line option ‘-fno-threadsafe-statics’ is valid for C++/ObjC++ but not for C