Fix T47377: Newer file crashes at render on official 2.76b version

Really annoying bug, the code was not forward compatible at all and
resulted in crash. And it is really good to keep at least one release
forward compatibility so possible regressions could be verified easily.

The idea now is to use new property name for the pixel filter type,
but keep old property around for a couple of releases, so we have at
least some forward compatibility.

Don't like this situation at all, but seems it's least of the evil
we can choose.

Thanks Brecht for the review!

intern/cycles/blender/addon/properties.py

@@ -376,12 +376,24 @@ class CyclesRenderSettings(bpy.types.PropertyGroup):
                 default=False,
                 )
 
+        # Really annoyingly, we have to keep it around for a few releases,
+        # otherwise forward compatibility breaks in really bad manner: CRASH!
+        #
+        # TODO(sergey): Remove this during 2.8x series of Blender.
         cls.filter_type = EnumProperty(
                 name="Filter Type",
                 description="Pixel filter type",
                 items=enum_filter_types,
                 default='BLACKMAN_HARRIS',
                 )
+
+        cls.pixel_filter_type = EnumProperty(
+                name="Filter Type",
+                description="Pixel filter type",
+                items=enum_filter_types,
+                default='BLACKMAN_HARRIS',
+                )
+
         cls.filter_width = FloatProperty(
                 name="Filter Width",
                 description="Pixel filter width",