Surface: Do not copy back if front and back buffer are identical

It is unnecessary to copy from and to the same surface. Skipping
the copy back is a huge performance improvement when using shared
buffer mode and locking a rectangle of the surface.

Bug: 38398135
Test: Use stylus demo and remove full-surface locking hack.
Change-Id: Ie8d03468507489247ba4a4be5f0cde5960e69c16