Add tkinter PhotoImage.redither method

[serhiy-storchaka]

The redither subcommand of the Tk photo image (since Tk 8.5) has no tkinter wrapper and is currently reachable only through tk.call().

imageName redither recalculates the dithered image in each window where it is displayed.
This is useful when the image data was supplied in pieces, in which case the dithered image may not be exactly correct.

I propose to add the corresponding PhotoImage.redither() method, completing the wrapping of the photo image subcommands.

Linked PRs

• gh-151888: Add tkinter PhotoImage.redither method #151889