Fix ridgemask

[Ichunjo]

• Integer clips have wrong output so the processing is now always in 32-bit float
• xy derivative was wrong (it didn't affect the output much)
• The expr was missing a 0.5 factor. Output will be different and is a breaking change.
• 5x5 convolutions need a right border fix. Technically, we only need 2 pixels but you'd have to fix each derivative and it slows down the process by quite a bit. Adding a 4 px mirror border and cropping it at the end is close enough.