Add mirror() to shapes

Might need to fix ordering on Text.to_polygons()

masque/shapes/shape.py

@@ -71,6 +71,16 @@ class Shape(metaclass=ABCMeta):
         """
         pass
 
+    @abstractmethod
+    def mirror(self, axis: int) -> 'Shape':
+        """
+        Mirror the shape across an axis.
+
+        :param axis: Axis to mirror across.
+        :return: self
+        """
+        pass
+
     @abstractmethod
     def scale_by(self, c: float) -> 'Shape':
         """