Make LOCAL_CONTEXT_KEY safe and non-leaky.

`LOCAL_CONTEXT_KEY` is currently a `Cell<*mut LocalLayoutContext>`. The use
of the raw pointer means that the `LocalLayoutContext` is not dropped when
the thread dies; this leaks FreeType instances and probably other
things. There are also some unsafe getter functions in `LayoutContext`
(`font_context`, `applicable_declarations_cache` and
`style_sharing_candidate_cache`) that @eddyb says involve undefined
behaviour.

This changeset changes `LOCAL_CONTEXT_KEY` to
`RefCell<Option<Rc<LocalLayoutContext>>>`. This fixes the leak and also
results in safe getters.

(Fixes #6282.)

components/layout/generated_content.rs

@@ -429,7 +429,7 @@ fn render_text(layout_context: &LayoutContext,
                                                              info));
     // FIXME(pcwalton): This should properly handle multiple marker fragments. This could happen
     // due to text run splitting.
-    let fragments = TextRunScanner::new().scan_for_runs(layout_context.font_context(), fragments);
+    let fragments = TextRunScanner::new().scan_for_runs(&mut layout_context.font_context(), fragments);
     debug_assert!(fragments.len() >= 1);
     fragments.fragments.into_iter().next().unwrap().specific
 }