Feature request: Tool to create combined image by gathering channels from input images

Use cases:

- RGB from one image, A from second image
- RG from one image, BA from second image. One (or both) of the images could be normal map that is encoded into two channels.

In theory also these could be possible:
- Four individual images, each providing single channel
- R and G from one image each, BA from third image (which could be normal map)

The issue names no files, entry points, or tests. Start by locating the existing KTX command-line tools and image/channel handling, then determine where a channel-combining tool belongs; done means the listed input combinations are supported and verified with representative images.

Written by the indexing model from the issue text.