Issue #12009: Restore drag arrows in Sample Colorize

This patch attempts to resolve #12009 (closed).

This feature was broken during the initial port to GimpProcedureConfig, due to the Out Level parameters not being properly clamped when interacted with.

This patch fixes the clamping and restores the call to level_in_draw () to show the arrows.

@ajonsson Hi! When you have time, feel free to test and let me know if there are any other issues. Thanks!