The walk that writes link annotations measured every object at every
level, on every export that can have links, and read the box only for
an object with a URL that opens and for a clipping group. It now
measures only those, so a document with no URLs or clipping groups is
not measured a second time. The link annotations are unchanged.
The output is byte for byte the same on a document of linked objects in
plain and clipping groups on and off the page, with default settings,
with uncompressed text and with PDF layers.
Each visible object with a URL (Attributes panel) is written as a Link
annotation with a URI action over its bounds on the page, trimmed to its
clipping groups. Hidden objects, non-printing layers and printed sheets get
none; PDF/X files leave them out with a warning. URIs are percent-encoded
to 7-bit ASCII.
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