Fix T41473: Cycles volume rendering is too dark

The issue was caused by the changed defaults from the Cycles side.
Because of those properties being saved as an IDProp and not being
saved to the file, every change to the defaults would ruin someone's
day updating the values.

Added a bpy.app.handler.version_update which is run after the regular
do_versions() are done and could be sued by the scripts to apply
versioning code on their settings.

Reviewers: campbellbarton

Reviewed By: campbellbarton

Differential Revision: https://developer.blender.org/D761

source/blender/python/intern/bpy_app_handlers.c

@@ -59,6 +59,7 @@ static PyStructSequence_Field app_cb_info_fields[] = {
 	{(char *)"scene_update_post", (char *)"Callback list - on updating the scenes data (after)"},
 	{(char *)"game_pre",          (char *)"Callback list - on starting the game engine"},
 	{(char *)"game_post",         (char *)"Callback list - on ending the game engine"},
+	{(char *)"version_update",    (char *)"Callback list - on ending the versioning code"},
 
 	/* sets the permanent tag */
 #   define APP_CB_OTHER_FIELDS 1