message: changes osx fallback symbol font style
  Previously, the fallback symbol font copied the style of the font from
  the config. However, the only available style for the fallback symbol
  font is Normal slant, Normal weight.
diff: @@ -164,7 +164,8 @@ impl ::Rasterize for Rasterizer {
  // fallbacks somehow.
  {
  let symbols = {
  - let d = FontDesc::new("Apple Symbols".to_owned(), desc.style.clone());
  + let fallback_style = Style::Description { slant:Slant::Normal, weight:Weight::Normal } ;
  + let d = FontDesc::new("Apple Symbols".to_owned(), fallback_style);
  self.get_font(&d, size)?
  };
  font.fallbacks.push(symbols);
  