To properly become a Dplug OwnedImage, Image should expose...

[p0nce]

• [x] a way to disown the Image data
• [x] a way to return separately the allocation pointer (free), and the first scanline position
• [x] a way to call the right free() function from Gamut for the pixel data
• [x] better semantics for null images, zero-size images, etc