fonts: Add FontIdentifier and LocalFontIdentifier (#31658)

Instead of using a simple `Atom` to identify a local font, use a data
structure. This allows us to carry more information necessary to
identify a local font (such as a path on MacOS). We need this for the
new version of WebRender, as fonts on MacOS now require a path.

This has a lot of benefits:
 1. We can avoid loading fonts without paths on MacOS, which should
    avoid a lot of problems with flakiness and ensure we always load the
    same font for a given identifier.
 2. This clarifies the difference between web fonts and local fonts,
    though there is more work to do here.
 3. This avoid a *lot* of font shenanigans, such as trying to work
    backwards from the name of the font to the path of the font we
    actually matched. In general, we can remove a lot of code trying to
    accomplish these shenanigans.
 4. Getting the font bytes always returns an `Arc` now avoiding an extra
    full font copy in the case of Canvas.

components/canvas/canvas_data.rs

@@ -501,8 +501,7 @@ impl<'a> CanvasData<'a> {
                         .first(font_context)
                         .expect("couldn't find font");
                     let font = font.borrow_mut();
-                    let template = font.handle.template();
-                    Font::from_bytes(Arc::new(template.bytes()), 0)
+                    Font::from_bytes(font.handle.template().bytes(), 0)
                         .ok()
                         .or_else(|| load_system_font_from_style(Some(style)))
                 })