Changed Kernel to subclass BuiltinFilter, instead of the other way around

Tests/test_image_filter.py

@@ -94,15 +94,15 @@ class TestImageFilter(PillowTestCase):
         self.assertEqual(rankfilter.size, 1)
         self.assertEqual(rankfilter.rank, 2)
 
+    def test_builtinfilter_p(self):
+        builtinFilter = ImageFilter.BuiltinFilter()
+
+        self.assertRaises(ValueError, builtinFilter.filter, hopper("P"))
+
     def test_kernel_not_enough_coefficients(self):
         self.assertRaises(ValueError,
                           lambda: ImageFilter.Kernel((3, 3), (0, 0)))
 
-    def test_kernel_filter_p(self):
-        kernel = ImageFilter.Kernel((2, 2), (0, 0, 0, 0))
-
-        self.assertRaises(ValueError, kernel.filter, hopper("P"))
-
     def test_consistency_3x3(self):
         source = Image.open("Tests/images/hopper.bmp")
         reference = Image.open("Tests/images/hopper_emboss.bmp")

src/PIL/ImageFilter.py

@@ -33,7 +33,14 @@ class MultibandFilter(Filter):
     pass
 
 
-class Kernel(MultibandFilter):
+class BuiltinFilter(MultibandFilter):
+    def filter(self, image):
+        if image.mode == "P":
+            raise ValueError("cannot filter palette images")
+        return image.filter(*self.filterargs)
+
+
+class Kernel(BuiltinFilter):
     """
     Create a convolution kernel.  The current version only
     supports 3x3 and 5x5 integer and floating point kernels.
@@ -60,16 +67,6 @@ class Kernel(MultibandFilter):
             raise ValueError("not enough coefficients in kernel")
         self.filterargs = size, scale, offset, kernel
 
-    def filter(self, image):
-        if image.mode == "P":
-            raise ValueError("cannot filter palette images")
-        return image.filter(*self.filterargs)
-
-
-class BuiltinFilter(Kernel):
-    def __init__(self):
-        pass
-
 
 class RankFilter(Filter):
     """