plugins/shakecursor: Don't trigger for cursor pushback

The shake detector is mistriggered by sporadic pointer movement when the
cursor is pushed back by a screen edge.

As a way to prevent it, this change adds a bounding rectangle
constraint. The cursor will move chaotically within a small bounding
rectangle if it's pushed back. So it should be enough to just check
the length of the diagonal.

src/plugins/shakecursor/shakedetector.cpp

@@ -72,7 +72,7 @@ std::optional<qreal> ShakeDetector::update(QMouseEvent *event)
     const qreal boundsWidth = right - left;
     const qreal boundsHeight = bottom - top;
     const qreal diagonal = std::sqrt(boundsWidth * boundsWidth + boundsHeight * boundsHeight);
-    if (diagonal == 0) {
+    if (diagonal < 100) {
         return std::nullopt;
     }