[HarfBuzz] Adding tracing

[Martin Hosken]

Dear All,

I've added tracing to harfbuzz-ng. This is to help font developers rather than programmers, figure out what is going on with their fonts. The cost is a simple if() for each lookup. If that is too high, we can probably make tracing optional.

Code available from gitorious.org:harfbuzz-dev/harfbuzz-dev.git (can't remember how to get it publicly).

The tracing goes all the way through to the python test code via a callback.

Yours,
Martin