message: Fix sign error in CoreText font rasterizer
  Descent was being reported as a positive value instead of negative. This
  caused the background and text alignment to be off dramatically.
  Resolves
diff: @@ -334,7 +334,7 @@ impl Font {
  Metrics {
  average_advance: average_advance,
  line_height: line_height,
  - descent: self.ct_font.descent() as f32,
  + descent: -(self.ct_font.descent() as f32),
  }
  }
  