Let engine instances in the same FlutterEngineGroup use the same font collection and thus share the various downstream glyph layout metrics data etc.

There's still seemingly a bug in https://docs.google.com/document/d/1wRl4vEbxgj7Q247xdxQ9hYDbiakT1g2dXmdprPIUatE/edit?ts=5f8881bc#heading=h.jw17g9alwqpy possibly relating to asset bundled custom fonts that would need addressing too.