libgui: Copyback all region except dirty region for newly allocated buffer

- Inside Surface::lock if new buffer is allocated and DirtyRegion does not
cover complete buffer bounds then copyback may not cover all remaining area
as it copyback only area covered by dirty regions from other buffers. This
will lead to left out black area which may cause flicker.

Change-Id: I4a3f7a56fc5fbaf4af926584919577d8d34bed57