Add constrained-copy partition to speed features.
Copy up to a certain bsize, otherwise set to a fixed bsize. This helsp to reduce artifact near moving boundary caused by full partition copy without checking motion of super-block. This artifact can occur at speeds 3,4 in real-time mode. Issue: https://code.google.com/p/webm/issues/detail?id=738. Change-Id: I05812521fd38816a467f72eb6a951cae4c227931