[HarfBuzz] harfbuzz coordinate system

[Martin Hosken]

Dear Behdad,

> I think I'm going to make that change, especially since everytime I hooked up
> HarfBuzz to another system, it worked the first try except for a missing
> negation sign for y_offset.

I don't think it matters too much which way you jump on this one so long as:

1. You clearly state the directionality in lots of places in the documentation and code.

2. You make it clear where the different y directionalities are used. For example, what should a call to a font metrics function return? If it is as per a font with y increasing up the page, then where in the code does the y directionality switch?

I sympathise with the quandary: font designers think in terms of y going up the page, while graphics programmers think of y increasing down the page.

Yours,
Martin