Fixed a full refresh for Clone layers

Now the Async Merger knows about one more type of layer position:
N_EXTRA. Such layer will be counted in a need rect chain and will be
updated (original+projection), but it will not be merged into the
final projection of the image. This is done for preliminary updates of
the sources of Clone layers.

Btw, i would suggest everyone to perform 'make clean' in
./krita/image/ folder, because many parts of walkers are
inlined. There is no need to do a full clean, just in ./image/
directory.