FlattenSimilarBlocks should only be tried when blending is possible.
FlattenSimilarBlocks should only be tried when blending is possible.

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.

Change-Id: I05612c952fdbd4b3a6e0ac9f3a7d49822f0cfb9b

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