evas png loader: optimize scale down logic.

evas png loader: optimize scale down logic.

Previously, mannual scale down logic was too primitive,
it copied pixel data each channels. Obviously, it's ineffective.

We know the general case - 4 bytes channel which is the most usage,
If loader copies data per four bytes, instructions could be reduced.

When I load scale-downed image(original 8k), about 0.02 secs was reduced by this.