Using tiles requires the "tile_size_on" parameter to be set to 1. Move the setting the 'default value' of this and another parameter to before we try to set it from Destination settings.
Allow the setting of file format Destination settings.
If passing a NULL profile to transform_colour*, use the image's profile (or the default generated one).
When converting a scan, use a Destination that has maximum/lossless file format settings. Also convert pixel format if the file format wants to (JP2file requires planar image data).