Evas filters: Rename RGBA_Image::mask.data into image.data8

Evas filters: Rename RGBA_Image::mask.data into image.data8

The structure should not be changed, despite the union modification.

I am renaming for consistency with older branches that had a mask
field in RGBA_Image. Also, the mask.data or data8 is really just
a way to avoid casting between DATA8 and DATA32 (and it shows
clearly what kind of data you are dealing with).