When the interpolated value is divided by the sum of weights, no
rounding is done, which means the value is truncated. This results in
a slight bias towards dark green in the interpolated area. Rounding
properly removes the bias.

I measured this change to reduce the interpolation error by 1 to 2 %
on average on a number of sample input and logo area combinations.
Signed-off-by: Jean Delvare <jdelvare@suse.de>
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>