Make popup image feature optional for now - #21017

I don't think the popup image feature is ready for mainstream use as of now.

• Kitty graphics protocol backend is designed wrong
• clipwindow property of popup images does not work well with images
• Clipping of images does not behave as expected
• Possibly other bugs

I plan on adding tests later:

• GTK4 GUI we can use the existing GTK4 infastructure for comparing render nodes. This should be feasible.
• For Kitty graphics protocol and sixel, we could possibly run a graphical terminal in a headless kiosk Wayland compositor (e.g. cage), take a screenshot of the output, and compare to an existing image with ImageMagick. Same with GTK3 GUI. Not sure if this will actually work however
• Not sure about MS-Windows (GDI), but it should be very similar to the GTK3 GUI in how its implemented. So not having tests shouldn't be too big of a deal.