api->retract_undo() added to Magic API; used by Filled Polygon

New API function which rolls back the Undo buffer (which is snapshotted
upon a mousedown event ahead of calling the Magic tool's `_click()`
function.)  Dangerous; use sparingly!

Filled Polygon uses this to prevent partially-created (aka unfinished
aka incomplete) polygons -- the preview lines and control points -- from
being recorded and wasting space in the Undo history, or reappearing
upon Redo.

TODO - Other tools (e.g., Clone) would certainly benefit from this.
TODO - Needs more testing to help identify any bugs/issues introduced
by this!
TODO - Magic API docs need updating.

This is probably a sufficient solution that will make
https://sourceforge.net/p/tuxpaint/feature-requests/246/,
'"Overlay" surface in Magic API', unnecessary.

docs/CHANGES.txt

@@ -6,7 +6,7 @@ Copyright (c) 2002-2024
 Various contributors (see below, and AUTHORS.txt)
 https://tuxpaint.org/
 
-2024.May.30 (0.9.33)
+2024.June.1 (0.9.33)
   * New Magic Tools:
     ----------------
     * Dither & Dither (Keep Color): Replaces all or part of a drawing
@@ -42,6 +42,12 @@ https://tuxpaint.org/
       + Idea from O'Hare The Rabbit
       + Coding by Bill Kendrick <bill@newbreedsoftware.com>
 
+    * Ability for Magic tools to retract an undo buffer snapshot
+      (`api->retract_undo()`).
+      + Used by new Filled Polygon magic tool.
+      + Note: `TP_MAGIC_API_VERSION` bumped to 0x0000000A.
+      + Bill Kendrick <bill@newbreedsoftware.com>
+
   * Improvements to Eraser tool:
     ----------------------------
     * Transparent erasers