Use the width of letter 'M' instead of font average width

The average character width in the font information is not always
reliable, so get the width from the character 'M' instead. It still
fall backs to the average if there's no 'M' in the font.

src/renderer/fonts/caching_shaper.rs

@@ -90,34 +90,50 @@ impl CachingShaper {
         let mut font_size = self.current_size();
         trace!("Using font_size: {:.2}px", font_size);
         self.font_loader = FontLoader::new(font_size);
-        let font_width = self.metrics().average_width;
-        trace!("Font width: {:.2}px", font_width);
+        let (metrics, font_width) = self.info();
+        trace!(
+            "Font width: {:.2}px {:.2}px",
+            font_width,
+            metrics.average_width
+        );
         self.fudge_factor = font_width.round() / font_width;
         trace!("Fudge factor: {:.2}", self.fudge_factor);
         font_size = self.current_size();
         trace!("Fudged font size: {:.2}px", font_size);
-        trace!("Fudged font width: {:.2}px", self.metrics().average_width);
+        trace!("Fudged font width: {:.2}px", self.info().1);
 
         self.font_loader = FontLoader::new(font_size);
         self.blob_cache.clear();
     }
 
-    fn metrics(&mut self) -> Metrics {
+    fn info(&mut self) -> (Metrics, f32) {
         let font_pair = self.current_font_pair();
         let size = self.current_size();
-        let shaper = self
+        let mut shaper = self
             .shape_context
             .builder(font_pair.swash_font.as_ref())
             .size(size)
             .build();
+        shaper.add_str("M");
+        let metrics = shaper.metrics();
+        let mut advance = metrics.average_width;
+        shaper.shape_with(|cluster| {
+            advance = cluster
+                .glyphs
+                .first()
+                .map_or(metrics.average_width, |g| g.advance);
+        });
+        (metrics, advance)
+    }
 
-        shaper.metrics()
+    fn metrics(&mut self) -> Metrics {
+        self.info().0
     }
 
     pub fn font_base_dimensions(&mut self) -> (u64, u64) {
-        let metrics = self.metrics();
+        let (metrics, glyph_advance) = self.info();
         let font_height = (metrics.ascent + metrics.descent + metrics.leading).ceil() as u64;
-        let font_width = metrics.average_width.round() as u64;
+        let font_width = (glyph_advance + 0.5).floor() as u64;
 
         (font_width, font_height)
     }