Subsequence caching might become a bottleneck of CompositeAfterPaint painting.

When one of many composited layers changed, in Pre-CompositeAfterPaint, , we only need to repaint the changed composited layer.

In CompositeAfterPaint, there are several potential problems:

1. we need to regenerate the whole paint artifact when anything changed in the whole page. Though with display item and subsequence caching, we don't need to repaint some display items/subsequences, we still need to copy them.

2. We also need to repaint the ancestors of the changed layer.

3. We need to add overlays (frame overlays and link highlights) after the painted result in the display item list. The overlays may change even if the page contents don't change and we still need to copy the whole paint artifact of the page contents.

One thought is to break the contiguous display item list into multiple pieces, each for one subsequence, then we only need to move a subsequence from the old paint artifact into the new paint artifact when the subsequence is cached.