Request: support transcoding via the CLI (demo app)

[gregbenz]

While the CLI tool (demo app) was probably originally just intended to show usage of the library, it has become a very useful tool on its own.

It would be ideal if the CLI tool would support transcoding an existing supported gain map with the following options:

• Crop, resize, and probably mirror (ie functions in the library which are not currently exposed in the CLI)
• all the other normal encode options (as if you were working from raw inputs like P010 rather than an existing gain map)

Doing this would make it trivial to transcode images with no additional coding or development. The lack of transcoding support is the key barrier to more widespread HDR growth and adoption and libultrahdr's CLI tool would be greatly beneficial in helping to address that.