SaveOptions::for_doc chose pretty-printed JSON by the object count in
the layers alone. A document with few layer objects but many anchors,
or with its art in symbols, patterns or graph designs, was therefore
written pretty, about ten times the compact size: every anchor's "p",
"in" and "out" spread over several indented lines.
A document is now pretty only when it has at most PRETTY_MAX_OBJECTS
objects and its compact JSON is at most PRETTY_MAX_BYTES (1 MiB). The
size is measured by serializing into a writer that stops at the limit,
so a large document costs at most 1 MiB of extra serializing. Small
documents stay readable and diffable.
Fixes#1484
37954523ae