part of fix for [2363362] [Sapphire] Corrupted raw picture displayed during snapshot

make sure to fallback properly to software when copybit operation fails.
with this change, the preview image will at least be displayed in b&w
(since GL doesn't support the yuv format). This would also fix
2363506, but that one is now handled more cleanly.
diff --git a/libs/surfaceflinger/LayerBuffer.h b/libs/surfaceflinger/LayerBuffer.h
index 90f83c4..1b31435 100644
--- a/libs/surfaceflinger/LayerBuffer.h
+++ b/libs/surfaceflinger/LayerBuffer.h
@@ -135,6 +135,7 @@
         virtual void destroy() { }
     private:
         status_t initTempBuffer() const;
+        void clearTempBufferImage() const;
         mutable Mutex                   mBufferSourceLock;
         sp<Buffer>                      mBuffer;
         status_t                        mStatus;