Add font metric caching

CHANGELOG.md

@@ -37,6 +37,7 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 - Stack overflow when printing shader creation error
 - Underline position for bitmap fonts
 - Selection rotating outside of scrolling region
+- Throughput performance problems caused by excessive font metric queries
 
 ### Removed
 

alacritty/src/renderer/mod.rs

@@ -341,9 +341,7 @@ impl GlyphCache {
     }
 
     pub fn font_metrics(&self) -> font::Metrics {
-        self.rasterizer
-            .metrics(self.font_key, self.font_size)
-            .expect("metrics load since font is loaded at glyph cache creation")
+        self.metrics
     }
 
     // Calculate font metrics without access to a glyph cache