[API] Allow negative font x_scale/y_scale

[API] Allow negative font x_scale/y_scale

I was reconsidering whether y should grow down, since all three/four
times I've used this API I was tricked and got that wrong in my use.
So I was very inclined to make y grow down instead of up.  However,
considering that the font space has y up and it would be very confusing
for callbacks to work against that, I decided that what I really want
is for the user to be able to set y_scale to a negative number to imply
that user-space y grows down.

Changing x_scale/y_scale from unsigned int to int allows that, and I've
made pango to use that instead of negating glyph y_offset later.  hb-ft
however still has y group up.  I *guess* that's how FreeType works?
I'm not sure, FreeType docs don't make this clear...

I'm happy with the resolution :-).