Evas filters: Implement color curves filter

Evas filters: Implement color curves filter

Color curves are a very simple tool to alter the colors of an image,
on a per-pixel basis. This implementation will simply map each pixel
to a 256 bytes buffer, provided by the application.

NOTE: There are no convenience functions yet for easy curve
generation, but this is the plan (give point A, B, B and interpolate
between them to generate the 256 values array).