Slightly better similarity samples

Slightly better similarity samples

Slight modification of the image_sim_fill_data, that creates the
similarity signature. When the image size is not a multiple of 32, all
cells had size w/32, so the last w%32 pixels were just ignored, and more
importantly it was not symmetric at all when the signature was built
from the other side, which is annoying for my comparison algorithm. So I
changed the cell size (x_inc) from a constant one to
"round(w_left/n_left)", in order to spread the spare pixels (eg to split
42 pixels into 4 cells, it results in 11,10,11,10 instead of
10,10,10,10). With an image of size w with w%32=16, my
rotation-invariant algorithm results in a similarity of
0.951000 with the original version, and 0.999968 with this modification
(apparently lossless jpeg rotation is not perfectly lossless).

Signed-off-by: Klaus Ethgen <Klaus@Ethgen.de>