When there are no more platform views, make sure the overlay layer is
hidden. When a platform view is added again, show the overlay. The
show/hide allows us to avoid continually recreating and destroying the
overlay surface + swapchain when a platform view slides in and out of
frame.
To further reduce memory usage, we could do a delayed de-allocation of
the overlay layer (say after 50 frames of no platform view, destroy it).
But I'm leaving this to a follow up.
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