[HarfBuzz] Adding debug support to hb

[Martin Hosken]

Dear Behdad,

As we approach a v1.0 I would like to encourage us to add debug support for font developers.

Looking at the rather nice hb_auto_trace, it would really help if the trace routine could be passed the buffer as well as the lookup being traced. That way a font debugger could show the glyph string state before and after a lookup runs, the position in the glyph string where the lookup is executed and which lookup executed.

Yours,
Martin