Fix sbix glyph extents

Fix sbix glyph extents

* The ‘height’ needs to be negated since the API returns “distance from
  top to bottom side”.
* Similarly, the ‘y_offset‘ needs to be added to the height to get the
  ‘y_bearing’, since sbix’s offset is “the point in the glyph relative
  to its lower-left corner which corresponds to the origin” while
  ‘y_bearing’ is the “top side of glyph from origin”.

With these changes the sbix glyph metrics return values similar to other
tables, as they were otherwise unusable.