impl AsRef<[u8]> for FontData (#5757)

* [x] I have followed the instructions in the PR template

This PR implements `AsRef<[u8]>` for `FontData`, allowing it to be
passed into `fontdb`'s
[`Source`](https://docs.rs/fontdb/0.16.2/fontdb/enum.Source.html) type.
This would allow `egui` and `cosmic_text` to share font data with
eachother

crates/epaint/src/text/fonts.rs

@@ -144,6 +144,12 @@ impl FontData {
     }
 }
 
+impl AsRef<[u8]> for FontData {
+    fn as_ref(&self) -> &[u8] {
+        self.font.as_ref()
+    }
+}
+
 // ----------------------------------------------------------------------------
 
 /// Extra scale and vertical tweak to apply to all text of a certain font.