message: Refactor darwin code
  The ascent calculation on darwin was more complicated than it needed to
  be. By running a `.ceil()` instead of adding one, checking if it's 0,
  substracting if it is, and then flooring it, a few instructions could be
  shaved off.
diff: @@ -475,13 +475,11 @@ impl Font {
  // Get the top of the bounding box
  let metrics = self.metrics();
  let height = metrics.line_height;
  - let mut ascent = height - self.ct_font.descent() + 1.;
  - if ascent.floor() == ascent {
  - // Fix off-by-one with an exact X.0 ascent
  - ascent -= 1.;
  - }
  + let ascent = (height - self.ct_font.descent()).ceil();
  +
  // Get the width of the cell
  let width = self.glyph_advance('0') as i32;
  +
  // Return the new custom glyph
  if character == super::BEAM_CURSOR_CHAR {
  return super::get_beam_cursor_glyph(ascent as i32, height as i32, width);
  