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

Even in libharfbuzz-subset, we still will handle our own threadsafety
for statics.

We use calloc() to allocate simple structs and set them to zero.
For bulk memory, use malloc() that doesn't zero.

Such that hmtx accelerator can be used from libharfbuzz-subset.

sanitizer.sanitize never return nullptr.

We want to use it instead of reinventing hashmap...

Also, don't change linker for ICU since libicu also uses libstdc++.