image stitching revisit

Problem

1. inter-FOV z diff is always present.
2. align them in image domain or audio ?

Analysis

1. If z un-aligned in image, later update_neighbor() filtering will transfer wrong z values to neighbor-FOVs that don't align with the focus-FOV.
2. Stitching in image registration gives rough result that's visually satisfied.
3. Stitching in audio registration yields aurally satisfied refined stitching.