#555 More accurate wrapMargin estimate

[Makarius]

The included patch for SVN trunk 23903 implements a more accurate estimate of wrapMargin. This is relevant for fractional font metrics and anti-aliasing. The effect is immediately visible by the "wrap guide", the vertical line at the text margin.

E.g. tested with Deja Vu Sans Mono 40 on Linux (GTK or Nimbus).

[Alan Ezust]

Committed Rev# 23912.