fix(iba): IBA::transpose() didn't set output image's format to input

[lgritz]

trafficstars

Because IBA::transpose changes resolution, it doesn't pass the input image to IBAPrep(), and there was a special case there that always set destination to float if there were no source images.

So change transpose to specifically request a format, to be the same as the input image.

And that required a tweak to IBAPrep allowing us to specify such a thing (there was already an option that meant "force it to float", but not the general case of an arbitrary type).