Move the rendering notifier into the Renderer. This duplicates a little bit of code, but it
(1) Allows for avoiding the flush of femtovg/skia if no rendering notifier is
registered
(2) Paves the way of gracefully handling the situation where the renderer doesn't support this yet
Resize the glutin context when we receive a resize event from winit.
This is the approach recommended by glutin/winit's documentation.
This approach also avoids jitter when rendering with metal (not
implemented yet, but tested locally :)
