Add text: watermarks, page numbers, Bates stamps

[tannevaled]

The other half of what people do to a PDF they have: write on it. One
primitive — a line of text, a face, a size, a colour, a transparency, an angle
and one of nine places — with the three things people actually ask for built on
it. The text may name what it is drawn on: {page}, {pages}, and {n} for a
counter that advances once per page, which is what a serial numbering scheme
needs.

Nothing is embedded. The four faces every viewer is required to have are enough
for a watermark or a page number, so a stamp costs about a kilobyte and works
everywhere. Their widths are Adobe's own, exact through printable ASCII, with a
Latin-1 letter taking the width of the letter it is built on — which is true of
these faces. Text is encoded as WinAnsi, and a character that encoding has no
room for is dropped rather than drawn as something else.

A stamped page keeps everything it had. Its own content streams are referenced
unchanged, wrapped in a save and a restore so whatever state they leave behind
cannot reach the text, and the resources are rebuilt as a dictionary of this
file's own — because the page may be sharing them with pages that are not being
stamped. The text goes in the page's own box, so on a page carrying a rotation
it turns with the page, as the page's own layout does. A page used as a tile is
wrapped first, so its text lands in the space the page is displayed in.

Verified across the corpus: for each of the 118 833 files, numbering every page
has to leave the page's own content stream in the file byte for byte, draw the
number, name a face the file carries, and put the text inside the page. All
118 833 hold — after the campaign found two real defects. A /Contents written
as a reference to an array, which 2 194 files do, lost the page's content
entirely; and 950 pages are smaller than the default margin, which put their
text off the page. Both are fixed, and the earlier 118 833-file and 1 959-file
checks still pass.

What this cannot yet check is what the text looks like. That needs a
rasteriser, and when there is one these stamps should be checked in pixels.

Exact 100% statement coverage over the package and the command, go vet clean,
nine cross-compilation targets.