[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 :-).

src/hb-font.cc

diff --git a/src/hb-font.cc b/src/hb-font.cc
index a84dde4..733eb3b 100644
--- a/src/hb-font.cc
+++ b/src/hb-font.cc
@@ -523,8 +523,8 @@
 
 void
 hb_font_set_scale (hb_font_t *font,
-		   unsigned int x_scale,
-		   unsigned int y_scale)
+		   int x_scale,
+		   int y_scale)
 {
   if (HB_OBJECT_IS_INERT (font))
     return;
@@ -535,8 +535,8 @@
 
 void
 hb_font_get_scale (hb_font_t *font,
-		   unsigned int *x_scale,
-		   unsigned int *y_scale)
+		   int *x_scale,
+		   int *y_scale)
 {
   if (x_scale) *x_scale = font->x_scale;
   if (y_scale) *y_scale = font->y_scale;

src/hb-font.h

diff --git a/src/hb-font.h b/src/hb-font.h
index 397b586..d9d6090 100644
--- a/src/hb-font.h
+++ b/src/hb-font.h
@@ -231,13 +231,13 @@
  */
 void
 hb_font_set_scale (hb_font_t *font,
-		   unsigned int x_scale,
-		   unsigned int y_scale);
+		   int x_scale,
+		   int y_scale);
 
 void
 hb_font_get_scale (hb_font_t *font,
-		   unsigned int *x_scale,
-		   unsigned int *y_scale);
+		   int *x_scale,
+		   int *y_scale);
 
 /*
  * A zero value means "no hinting in that direction"

src/hb-ot-layout-common-private.hh

diff --git a/src/hb-ot-layout-common-private.hh b/src/hb-ot-layout-common-private.hh
index 9ff5ca9..00a1432 100644
--- a/src/hb-ot-layout-common-private.hh
+++ b/src/hb-ot-layout-common-private.hh
@@ -532,7 +532,7 @@
   inline hb_position_t get_y_delta (hb_ot_layout_context_t *c) const
   { return get_delta (c->font->y_ppem, c->font->y_scale); }
 
-  inline int get_delta (unsigned int ppem, unsigned int scale) const
+  inline int get_delta (unsigned int ppem, int scale) const
   {
     if (!ppem) return 0;
 
@@ -540,10 +540,6 @@
 
     if (!pixels) return 0;
 
-    /* pixels is at most in the -8..7 range.  So 64-bit arithmetic is
-     * not really necessary here.  A simple cast to int may just work
-     * as well.  But since this code is not reached that often and
-     * for the sake of correctness, we do a 64bit operation. */
     return pixels * (int64_t) scale / ppem;
   }
 

src/hb-ot-layout-private.hh

diff --git a/src/hb-ot-layout-private.hh b/src/hb-ot-layout-private.hh
index a802c63..b032a7a 100644
--- a/src/hb-ot-layout-private.hh
+++ b/src/hb-ot-layout-private.hh
@@ -77,7 +77,7 @@
   inline hb_position_t scale_y (int16_t v) { return scale (v, this->font->y_scale); }
 
   private:
-  inline hb_position_t scale (int16_t v, unsigned int scale) { return v * (int64_t) scale / this->face->head_table->get_upem (); }
+  inline hb_position_t scale (int16_t v, int scale) { return v * (int64_t) scale / this->face->head_table->get_upem (); }
 };