Set lastSourceNode when serializing children + related fixes.

* When children are being serialized recursively, state sets up
  a new callback. lastSourceNode and lastSourceSep should be set
  up in that callback -- was missing.

  This fix lets us RT this snippet accurately:
  [[File:foo.jpg|thumb|300px|foo\n{{echo|A}}\n{{echo|B}}\n{{echo|C}}]]

* Added DSR tag widths for figure-tags since we know what they are.

* Improved ability to emit source-based seps in the (lastChild, parent)
  separator pair scenario.

  These two fixex lets us RT this snippet accurately:
  [[File:foo.jpg|thumb|300px|foo\n{{echo|A}}\n{{echo|B}}\n{{echo|C}}\n\n]]

* No change in parser test results (TODO: Add new tests)

* Eliminates RT diffs from en:Roman Times

Change-Id: I47c242ba4b8dc011ff7b41355cd8775da777e9ad