Python:
* adds bpy.app.handlers which contains lists, each for an event type:
render_pre, render_post, load_pre, load_post, save_pre, save_post
* each list item needs to be a callable object which takes 1 argument (the ID).
* callbacks are cleared on file load.
Example:
def MyFunc(scene): print("Callback:", data)
bpy.app.handlers.render_post.append(MyFunc)
C:
* This patch adds a generic C callback api which is currently only used by python.
* Unlike python callbacks these are not cleared on file load.
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