backport Ryg fix for vertical shifts

image/gfm/image/stb_truetype.d

@@ -745,15 +745,25 @@ void stbtt_FreeShape(const stbtt_fontinfo *info, stbtt_vertex *v)
 
 void stbtt_GetGlyphBitmapBoxSubpixel(const stbtt_fontinfo *font, int glyph, float scale_x, float scale_y,float shift_x, float shift_y, int *ix0, int *iy0, int *ix1, int *iy1)
 {
-   int x0,y0,x1,y1;
-   if (!stbtt_GetGlyphBox(font, glyph, &x0,&y0,&x1,&y1))
-      x0=y0=x1=y1=0; // e.g. space character
-   // now move to integral bboxes (treating pixels as little squares, what pixels get touched)?
-   if (ix0) *ix0 =  ifloor(x0 * scale_x + shift_x);
-   if (iy0) *iy0 = -iceil (y1 * scale_y + shift_y);
-   if (ix1) *ix1 =  iceil (x1 * scale_x + shift_x);
-   if (iy1) *iy1 = -ifloor(y0 * scale_y + shift_y);
+    int x0, y0, x1, y1;
+    if (!stbtt_GetGlyphBox(font, glyph, &x0, &y0, &x1, &y1)) 
+    {
+        // e.g. space character
+        if (ix0) *ix0 = 0;
+        if (iy0) *iy0 = 0;
+        if (ix1) *ix1 = 0;
+        if (iy1) *iy1 = 0;
+    } 
+    else 
+    {
+        // move to integral bboxes (treating pixels as little squares, what pixels get touched)?
+        if (ix0) *ix0 = ifloor( x0 * scale_x + shift_x);
+        if (iy0) *iy0 = ifloor(-y1 * scale_y + shift_y);
+        if (ix1) *ix1 = iceil( x1 * scale_x + shift_x);
+        if (iy1) *iy1 = iceil(-y0 * scale_y + shift_y);
+    }
 }
+
 void stbtt_GetGlyphBitmapBox(const stbtt_fontinfo *font, int glyph, float scale_x, float scale_y, int *ix0, int *iy0, int *ix1, int *iy1)
 {
    stbtt_GetGlyphBitmapBoxSubpixel(font, glyph, scale_x, scale_y,0.0f,0.0f, ix0, iy0, ix1, iy1);
@@ -1009,9 +1019,9 @@ void stbtt__rasterize(stbtt__bitmap *result, stbtt__point *pts, int *wcount, int
             a=j,b=k;
          }
          e[n].x0 = p[a].x * scale_x + shift_x;
-         e[n].y0 = p[a].y * y_scale_inv * vsubsample + shift_y;
+         e[n].y0 = (p[a].y * y_scale_inv + shift_y) * vsubsample;
          e[n].x1 = p[b].x * scale_x + shift_x;
-         e[n].y1 = p[b].y * y_scale_inv * vsubsample + shift_y;
+         e[n].y1 = (p[b].y * y_scale_inv + shift_y) * vsubsample;
          ++n;
       }
    }