The raw pointer isn't a reliable id as the allocator can reuse an
address that's just been released for another layer.
This will fix Fuchsia bug FL-216.
This problem was not affecting non-Fuchsia Flutter probably because
non-Fuchsia Flutter purges the raster cache key much more frequently so
we won't see a key collision. In Fuchsia, as the key has to wait for the
Vulkan surface to render asynchronously, this suddenly becomes an issue.
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