This means that in RasterImage::LookupFrame when we are asked to do a sync decode (if needed) we use WaitUntilComplete to wait until the frame is finished decoding. But we would actually return after the next progressive pass notified the monitor to wake up. Thus, we would draw a not-fully-decoded image even though the sync decode flag was passed.
The change in FrameAnimator means that we won't draw the next frame in an animated image until all progressive passes of that image are complete. This seems like what we want anyways.
There is one real use of IsImageComplete left, in imgFrame::Draw, where we need to know if the decoded image data covers the whole image frame. (There are a couple of uses of IsImageComplete in asserts.)
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