Support @~ (Symbol font toggle) in -Sq quoted line labels

[joa-quim]

The @~ escape sequence for switching to the Symbol font (Greek letters
etc.) was ignored in -Sq quoted line labels. The label text was rendered
via PostScript's show operator in PSL_label.ps, which doesn't understand
GMT's @~ escape sequences.

Add PostScript procedures (PSL_split_esc, PSL_has_esc, PSL_show_esc,
PSL_charpath_esc, PSL_sw_esc) that split label strings on @~ boundaries
and toggle between the current font and Symbol font for each segment.
Wrapper functions (PSL_label_show, PSL_label_charpath, PSL_label_sw)
dispatch to the escape-aware versions only when @~ is present, so labels
without escapes follow the exact same code path as before.

For curved text (+v), modify setchar to detect @~ at the current position
and toggle fonts without rendering, allowing the next real character to
be placed in the correct font along the path.

Fixes both straight baseline labels (PSL_ST_place_label*) and curved
baseline labels (PSL_pathtext/setchar), as well as width calculations
(PSL_ST_prepare_text, PSL_CT_calcstringwidth).

Co-Authored-By: Claude Opus 4.6 noreply@anthropic.com"

[Esteban82]

I tested it with this script and it worked:

cat > flights.txt << END
> "Buenos Aires to Frankfurt - 11483 km in 12:24h"
-58.54 -34.82
  8.57  50.03
> "Frankfurt to Johannesburg - 8658 km in 9:21h"
  8.57  50.03
 28.24 -26.13
END

gmt begin symbols png
  gmt coast -JM0/20c -R-87/50/-57/60 -W0.5p,grey43 -Sgrey88 -A300 -B
  gmt plot flights.txt -W5p,red --PS_LINE_CAP=round \
    -Sqn1:+f12p,Helvetica-Bold+l"@~p@~=0.5"
gmt end