Gradiant orientation and center

[Sleuthgame]

Add a step (mouse click ?) to choose freely gardient orientation and center.

For a etude of gradiants, I found thoses:
study https://expediteapps.net/2021/11/23/gradients/ interesting.
and
https://expiredpopsicle.com/articles/2021-08-09-Deluxe_Paint_Gradient_Fills/Deluxe_Paint_Gradient_Fills.html