Cycles: Only use new light sample threshold for new files

This is a late follow-up commit to the light sample threshold changes which
caused difference in rendering all existing .blend files which is not something
we are happy about: it is fine to use new optimized defaults for new files, but
existing ones should always be rendering in the same way as they used to be.

Sorry for the inconveniece, but such thing should have been done to begin with.
If this setting was modified it will not be reset to zero.

Now all render tests should be passing again.

P.S. Also really annoying to bump subversion for such reasons, but currently we
don't have better way to achieve what we want.

intern/cycles/blender/addon/version_update.py

@@ -278,3 +278,9 @@ def do_versions(self):
                     cscene.pixel_filter_type = cscene.filter_type
                 if cscene.filter_type == 'BLACKMAN_HARRIS':
                     cscene.filter_type = 'GAUSSIAN'
+
+    if bpy.data.version <= (2, 78, 2):
+        for scene in bpy.data.scenes:
+            cscene = scene.cycles
+            if not cscene.is_property_set("light_sampling_threshold"):
+                cscene.light_sampling_threshold = 0.0

source/blender/blenkernel/BKE_blender_version.h

@@ -28,7 +28,7 @@
  * and keep comment above the defines.
  * Use STRINGIFY() rather than defining with quotes */
 #define BLENDER_VERSION         278
-#define BLENDER_SUBVERSION      1
+#define BLENDER_SUBVERSION      2
 /* Several breakages with 270, e.g. constraint deg vs rad */
 #define BLENDER_MINVERSION      270
 #define BLENDER_MINSUBVERSION   6