evas png: apply interpolation when scale down image loading.
This patch improves png quality when image uses scale-down at image loading.
Since current scale-down logic just works like point sampling,
image result could be wholely different,
Simply, if source data is consist of continous white and black pixels,
and scale down factor is 2, the sampled data would be only white,
and lose all black pixels, or vice versa.
The result can be unexpected by users.
Even current jpeg scale-down works with interpolation.
Reviewers: cedric, raster, committers, kimcinoo, jsuya
Differential Revision: https://phab.enlightenment.org/D8788