Carry a picture's hyperlink into the PDF as a link over the image

[ikashapov]

Carry a picture's hyperlink into the PDF as a link over the image

Closes #250.

§21.1.2.3.5: Word stores a link added to a picture as <a:hlinkClick r:id="…"/> inside <wp:docPr>, resolved through the part's relationships like any other external hyperlink; LibreOffice wraps the run holding the drawing in <w:hyperlink> instead. Neither reached the PDF: the docPr child was accepted and dropped, and an image fragment did not inherit the enclosing hyperlink the way the text fragments beside it do.

Change

• DocProperties carries the target. It is parsed as ExternalRel; the body-level resolve_hyperlinks pass and the subordinate-part rel rewrite turn it into the final URL exactly as they do for w:hyperlink. A relationship whose target is a bare #fragment names a bookmark in this document and becomes a GoTo.
• Fragment::Image and FloatingImage carry a LinkTarget. Every site that draws a picture — the line, a wrapTopAndBottom float, a wrapped float, and the two cell-stacker paths — puts one annotation over exactly the drawn rectangle. A picture's own link wins over a w:hyperlink around its run.

Not read, deliberately: v:shape/@href (the VML spelling) and a:hlinkClick on pic:cNvPr, which Word writes alongside the docPr one.

Tests

tests/image_hyperlinks.rs, each drawing a 1×1 PNG and asserting an annotation covering exactly the image rectangle:

• a:hlinkClick on an inline picture → LinkAnnotation over the image;
• a picture inside w:hyperlink inherits the link;
• a:hlinkClick on an anchored picture follows the float;
• a relationship targeting #Top → InternalLink;
• control: a picture without a link gets no annotation.

Four of five watched fail before the change.