Shape tool improvements

More sensible rotation when using the drag-from-corner mode
(rotates around center of the shape, as you'd expect, rather than
around one of the corners of the shape's bounding box, as it has
been doing all this time).

Stretching a shape from bottom to top causes an upside-down shape
(e.g., before, a triangle would always start out up-pointing
regardless as to whether you clicked and drag downwards or upwards;
now, it will start out down-pointing if you drag upwards when
initially placing & stretching the shape).

docs/CHANGES.txt

@@ -7,7 +7,7 @@ Various contributors (see below, and AUTHORS.txt)
 https://tuxpaint.org/
 
 
-2023.March.3 (0.9.29)
+2023.March.6 (0.9.29)
   * Improvements to "Stamp" tool:
     -----------------------------
     * Stamps may now be rotated.
@@ -100,6 +100,18 @@ https://tuxpaint.org/
       (Based on http://www.codersnotes.com/notes/signed-distance-fields/
       by Richard Mitton)
 
+  * Improvements to Shape tool:
+    ---------------------------
+    * Rotation of corner-stretched shapes are around their center,
+      rather than orbiting a corner.
+      Bill Kendrick <bill@newbreedsofware.com>
+
+    * Stretching a shape upwards rotates it 180-degrees
+      (e.g., stretching the triangle downwards makes an up-pointing
+      triangle (^), as before; but stretching it upwards now makes
+      a down-pointing triangle (v))
+      Bill Kendrick <bill@newbreedsofware.com>
+
   * Improvements to Color selection:
     --------------------------------
     * The rainbow palette color picker allows you to switch to the