Add a flag to hb_buffer_t to prevent the insertion of dotted circles on incorrect character sequences. 

Current behavior unchanged if this flag is not set (and it isn't by default).

That function checks for buffer->successful already.  No need
to check at call site.

Show up with gcc -O0.

There's a few more but those are functions that need to be filled in.

Maybe this is a lost battle...

One of the was missed before.  Not intentional.

https://github.com/harfbuzz/harfbuzz/pull/1273