Fix warning

c:\projects\harfbuzz\src\hb-ot-color-cbdt-table.hh(59): warning C4146: unary minus operator applied to unsigned type, result still unsigned [C:\projects\harfbuzz\build\harfbuzz.vcxproj]

src/hb-ot-color-cbdt-table.hh

@@ -56,7 +56,7 @@ struct SmallGlyphMetrics
     extents->x_bearing = bearingX;
     extents->y_bearing = bearingY;
     extents->width = width;
-    extents->height = -height;
+    extents->height = - (hb_position_t) height;
   }
 
   HBUINT8	height;