Added get_vary and get_vary_key to Filter

This allows a filter operation to specify which strings of data it used
inside the image to perform its calculations. This can be used to
determine whether an image has changed in such a way that requires new
renditions to be made.

wagtail/wagtailimages/image_operations.py

@@ -147,6 +147,21 @@ class FillOperation(object):
 
         willow.resize(width, height)
 
+    def get_vary(self, image):
+        focal_point = image.get_focal_point()
+
+        if focal_point is not None:
+            focal_point_key = "%(x)d-%(y)d-%(width)dx%(height)d" % {
+                'x': int(focal_point.centroid_x),
+                'y': int(focal_point.centroid_y),
+                'width': int(focal_point.width),
+                'height': int(focal_point.height),
+            }
+        else:
+            focal_point_key = ''
+
+        return [focal_point_key]
+
 
 class MinMaxOperation(object):
     def __init__(self, method, size):

wagtail/wagtailimages/models.py

@@ -1,4 +1,5 @@
 import os.path
+import hashlib
 import re
 
 from six import BytesIO, text_type
@@ -428,6 +429,22 @@ class Filter(models.Model):
 
         return output
 
+    def get_vary(self, image):
+        vary = []
+
+        for operation in self.operations:
+            if hasattr(operation, 'get_vary'):
+                vary.extend(operation.get_vary(image))
+
+        return vary
+
+    def get_vary_key(self, image):
+        vary_string = '-'.join(self.get_vary(image))
+        vary_key = hashlib.sha1(vary_string.encode('utf-8')).hexdigest()
+
+        return vary_key[:8]
+
+
     _registered_operations = None
 
     @classmethod