Do gamma correction

[theotherphil]

https://www.reddit.com/r/rust/comments/d4inkc/imagecli_a_pure_rust_image_processing_command/f0e4u80?utm_source=share&utm_medium=web2x

imageproc already has an issue for this: image-rs/imageproc#237.

However, this use case is probably easier - we're in a slightly better position to work out what format an image is in, and apply appropriate conversions at the start and end of the pipeline.