"Smooth Rainbow" magic tool (variant of "Rainbow")

Numerous people commented on Twitter that a piece of artwork
("Chromie Squiggle #7583", created in "ArtBlocks") looked like it
was made in Tux Paint.

However, the classic (2002-era) "Rainbow" tool cycled coarsely
through solid colors.  This adds a gradient.

magic/magic-docs/en/smooth_rainbow.txt

@@ -0,0 +1,9 @@
+                     Tux Paint "Magic" Tool: Smooth Rainbow
+
+                  By Bill Kendrick <bill@newbreedsoftware.com>
+
+   This is similar to Rainbow magic tool — as you move the mouse around, it
+   cycles through a spectrum of colors — except the colors blend smoothly as
+   you draw.
+
+   See also: Rainbow