This is because, FlattenSimilarBlocks() replaces some opaque pixels by
transparent ones. This results in an equivalent output only if blending
is turned on for the current frame.
(cherry picked from commit 5cccdadf2e75e4eae684951fb507b2bebbda66cb)
Change-Id: I05612c952fdbd4b3a6e0ac9f3a7d49822f0cfb9b