Make interpolation algorithm more flexible

Right now it appears that reprojecting by interpolation is _only_ available as polynomial interpolation. Is that an intended restriction? For example, I would love to see Lanczos resampling (due to it's excellent smoothness and theoretically near-ideal S/N properties). But I presume other people have preferred algorithms they might also want to use?

Start by locating the existing polynomial interpolation path used during reprojection and trace how its algorithm is selected. Compare the requested Lanczos resampling with the current behavior, then clarify which algorithms and API shape should be supported. Done requires an agreed flexible interpolation design and tests covering the selected algorithms.

Written by the indexing model from the issue text.