v2.10.2 — Every SVG label is well-formed XML

A fix to SVG export, from #119. Output changes only for labels holding one
of the characters below; the conformance signature is unchanged.

Fixed

• SVG export escapes < and & before a combining mark, and replaces
  what XML can't hold. SVGRenderer escaped a label one Character at a
  time, but < or & with a combining mark after it (a ́, decoded
  since 2.8) is one Character, and it isn't <; it was written raw. A C0
  control other than tab, LF and CR, or U+FFFE / U+FFFF, typed or pasted
  into a label (an ANSI escape copied from a terminal), reached <text> as
  it was, and XML 1.0 can't hold one even as a reference. Either made the
  document unparseable. Escaping now walks scalars, and writes U+FFFD for
  those characters. Reported, with the fix, by @Joseph-Cursio.