Codechange: simplify UpdateCursorPositionRelative

The function is only called with fix_at=true, so don't support
the other cases.

src/gfx.cpp

@@ -1880,27 +1880,17 @@ void SetAnimatedMouseCursor(const AnimCursor *table)
 }
 
 /**
- * Update cursor position on mouse movement for relative modes.
+ * Update cursor position based on a relative change.
+ *
  * @param delta_x How much change in the X position.
  * @param delta_y How much change in the Y position.
  */
 void CursorVars::UpdateCursorPositionRelative(int delta_x, int delta_y)
 {
-	if (this->fix_at) {
-		this->delta.x = delta_x;
-		this->delta.y = delta_y;
-	} else {
-		int last_position_x = this->pos.x;
-		int last_position_y = this->pos.y;
-
-		this->pos.x = Clamp(this->pos.x + delta_x, 0, _cur_resolution.width - 1);
-		this->pos.y = Clamp(this->pos.y + delta_y, 0, _cur_resolution.height - 1);
+	assert(this->fix_at);
 
-		this->delta.x = last_position_x - this->pos.x;
-		this->delta.y = last_position_y - this->pos.y;
-
-		this->dirty = true;
-	}
+	this->delta.x = delta_x;
+	this->delta.y = delta_y;
 }
 
 /**