Fix block cursor does not render italic for vim (#10249)

Release Notes:

- Fixed #8799

crates/gpui/src/text_system.rs

@@ -117,6 +117,17 @@ impl TextSystem {
         }
     }
 
+    /// Get the Font for the Font Id.
+    pub fn get_font_for_id(&self, id: FontId) -> Option<Font> {
+        let lock = self.font_ids_by_font.read();
+        lock.iter()
+            .filter_map(|(font, result)| match result {
+                Ok(font_id) if *font_id == id => Some(font.clone()),
+                _ => None,
+            })
+            .next()
+    }
+
     /// Resolves the specified font, falling back to the default font stack if
     /// the font fails to load.
     ///

crates/gpui/src/text_system/line_layout.rs

@@ -103,6 +103,18 @@ impl LineLayout {
         self.width
     }
 
+    /// The corresponding Font at the given index
+    pub fn font_id_for_index(&self, index: usize) -> Option<FontId> {
+        for run in &self.runs {
+            for glyph in &run.glyphs {
+                if glyph.index >= index {
+                    return Some(run.font_id);
+                }
+            }
+        }
+        None
+    }
+
     fn compute_wrap_boundaries(
         &self,
         text: &str,