Image.scanline

Returns a pointer to the y nth line of pixels. Only possible if the image has plain pixels. What pixel format it points to, depends on the image type().

Guarantees by layout constraints:
 * next scanline (if any) is returned pointer + pitchInBytes() bytes.
 * scanline pointer are aligned by given scanline alignment flags (at least).
 * after each scanline there is at least a number of trailing pixels given by layout flags
 * scanline pixels can be processed by multiplicity given by layout flags
 * around the image, there is a border whose width is at least the one given by layout flags.

For each scanline pointer, you can _always_ READ ptr[0..pitchInBytes()] without memory error. However, WRITING to this scanline doesn't guarantee anything by itself since the image could be a sub-image, and the underlying buffer could be shared.