Improvements and fixes to incremental baking. * Better handling of the render pass during page rendering. * Used sources are paired with the pass they were used in. * Proper use and invalidation of the rendered segments cache based on render passes. * The `Assetor` is also better tracking what was used in a page. * Add flags on a page to get better caching information for the debug window. * Property invalidation of the previous bake record when needed. * Better information about why pages are delayed.

class Linker(object):
    def __init__(self, page):
        self.page = page