Don't paint the System Preview badge if the image isn't fully decoded
<rdar://problem/40454866>
We paint the system preview badge even if it hasn’t been decoded yet. It leads to flashes of black if the image hasn’t been decoded before, or a flash of the previous blurred backdrop (when pinch zooming in).
Created attachment 341688 [details] Patch
Committed r232367: <https://trac.webkit.org/changeset/232367>