Zoom/Perspective: Support background color changes

When in the middle of adjusting a drawing with Zoom or Perspective,
you could change the color.  However, the background would not
relfect this until you further adjusted the zoom/perspective.

Now, a color change causes the background to update immediately.

This involved updating the Tux Paint Magic API to send more
arguments to each Magic tool's "XYZ_set_color()" function,
which means a bump in the API version.

(Forthcoming, all other Magic tools will have their set_color()
functions updated.)

docs/CHANGES.txt

@@ -36,6 +36,10 @@ https://tuxpaint.org/
     * Better performance from "Rush" (by using SDL_gfx rotozoom)
       Pere Pujal i Carabantes <perepujal@gmail.com>
 
+    * "Zoom" and "Perspective" background colors change
+      immediately when you select new colors.
+      Bill Kendrick <bill@newbreedsoftware.com>
+
   * Other Improvements:
     -------------------
     * A keyboard shortcut is now available for quickly accessing
@@ -73,6 +77,11 @@ https://tuxpaint.org/
     * Changed default UI font from Bitstream Vera to DejaVu Sans.
       h/t Mark Kim <markuskimius@gmail.com>
 
+    * Update Tux Paint Magic API so `set_color()` functions can
+      alter the canvas.  (Utilized by "Zoom" and "Perspective", so far.)
+      Note: Bumps `TP_MAGIC_API_VERSION` to 0x00000007.
+      Bill Kendrick <bill@newbreedsoftware.com>
+
   * Bug Fixes:
     ----------
     * Always creating 24-bit canvases, in an attempt to avoid blending