Replace MaxRGB with QuantumRange
Work when QuantumDepth=16
Fix for #2843, doesn't work when QuantumDepth != 8
Rewrite to do mostly direct access to pixels
Avoid using PlusCompositeOp, which doesn't produce the desired
composite in 6.0.0.
Be helpful: write a descriptive message to stdout when invoked.
Rewritten and much expanded.
Remove shebang line