[HarfBuzz] Why does kerning modify glyph advance?

[Tom Hacohen]

Dear all,

Hey, there's a question I've been meaning to ask for a long time now,
why does kerning modify the x_advance and not the x_offset? The way I
see it, with this behavior x_advance of a char depends on it's position
which eliminates the possibility of caching it, while putting it in
x_offset (which is already position dependent) will allow it.

I bet you guys have a good reasoning (standard maybe? no idea..) for
this, please let me know.

Thanks in advance,
Tom.