retrace: Remove the -c/--compare=PREFIX .

That is, remove the ability to compare images while replaying.

For several reasons:

- This duplicates diff-images command.

- It is less versatile -- it is unable to output diff images.

- It will be necessary to change how snapshots are done to properly
  accommodate APIs that have no concept of current context, like
  Direct3D APIs, so the less fat on this code the better.