CoreText renderer is now the default on macOS

I've added dingbats to the fallback list on macos as that is needed
for some symbols we use in tools at work.

With that, and some tweaks for allowing for missing letter glyphs
when loading a symbol font as a fallback, we can make that the
default for macOS

Closes https://github.com/wez/wezterm/issues/5

src/config.rs

@@ -208,6 +208,12 @@ impl TextStyle {
             bold: None,
             italic: None,
         });
+        #[cfg(target_os = "macos")]
+        font.push(FontAttributes {
+            family: "Zapf Dingbats".into(),
+            bold: None,
+            italic: None,
+        });
         #[cfg(windows)]
         font.push(FontAttributes {
             family: "Segoe UI".into(),

src/font/coretext.rs

@@ -108,7 +108,7 @@ fn glyph_index(ct_font: &CTFont, codepoint: char) -> Option<CGGlyph> {
     }
 }
 
-#[derive(Debug)]
+#[derive(Debug, Default)]
 struct Metrics {
     font_metrics: FontMetrics,
     ascent: f64,
@@ -150,8 +150,9 @@ impl CoreTextFontImpl {
         let w_metrics = metrics('W', &ct_font);
         let m_metrics = metrics('M', &ct_font);
         let zero_metrics = metrics('0', &ct_font);
-        let metrics =
-            w_metrics.unwrap_or_else(|| m_metrics.unwrap_or_else(|| zero_metrics.unwrap()));
+        let metrics = w_metrics.unwrap_or_else(|| {
+            m_metrics.unwrap_or_else(|| zero_metrics.unwrap_or_else(|| Default::default()))
+        });
         Self {
             ct_font,
             hb_font,

src/font/mod.rs

@@ -58,10 +58,12 @@ impl Default for FontSystemSelection {
             not(feature = "force-rusttype")
         )) {
             FontSystemSelection::FontConfigAndFreeType
-        } else if cfg!(not(feature = "force-rusttype")) {
-            FontSystemSelection::FontLoaderAndFreeType
-        } else {
+        } else if cfg!(feature = "force-rusttype") {
             FontSystemSelection::FontLoaderAndRustType
+        } else if cfg!(target_os = "macos") {
+            FontSystemSelection::CoreText
+        } else {
+            FontSystemSelection::FontLoaderAndFreeType
         }
     }
 }
@@ -104,7 +106,7 @@ impl std::str::FromStr for FontSystemSelection {
     fn from_str(s: &str) -> Result<Self, Self::Err> {
         match s.to_lowercase().as_ref() {
             "fontconfigandfreetype" => Ok(FontSystemSelection::FontConfigAndFreeType),
-            "fontloadandfreetype" => Ok(FontSystemSelection::FontLoaderAndFreeType),
+            "fontloaderandfreetype" => Ok(FontSystemSelection::FontLoaderAndFreeType),
             "fontloaderandrusttype" => Ok(FontSystemSelection::FontLoaderAndRustType),
             "coretext" => Ok(FontSystemSelection::CoreText),
             _ => Err(format_err!(