Image_ansi_colors: dithering to the 256-colour terminal palette, for a terminal without true colour (from dev 1ce461b4)

sks/image/image.k

@@ -19,6 +19,19 @@
                colours per cell under ansi), braille (eight dots per cell).
          :value halfblock
 @@@
+@@@state Image_ansi_colors
+         :desc Under ansi, the colours an image is drawn with. true (the
+               default) emits each cell's exact 24-bit colour, and how
+               faithfully the picture appears then depends on the terminal
+               supporting true colour: most do (GNOME Terminal, iTerm2,
+               Ghostty, kitty, WezTerm); one that does not (Apple's
+               Terminal) rounds every colour to its nearest of 256 fixed
+               ones, and runs of similar cells become one blotch. 256 is
+               for such a terminal: the picture is first dithered to that
+               256-colour palette, so the rounding error is spread over
+               neighbouring cells instead. A true-colour terminal shows the
+               exact picture and needs no setting.
+         :value true @@@
 @@@state Image_txt_dark_background
          :desc Under txt and ansi, the density ramp assumes a dark terminal
                background (true): bright pixels are the dense characters.