Support LaTeX expressions in subplot panel tags

[Esteban82]

Description of proposed changes

LaTeX expressions in a subplot panel tag (subplot set -A"@[...@[") are not plotted. This is the request in #7958 and the forum post. This PR replaces the WIP in #7958.

The tag is plotted inside the PSL_plot_completion PostScript procedure, and the EPS made from the LaTeX expression is too large to fit in a procedure (Ghostscript limitcheck).

Changes:

• postscriptlight.c/h: new PSL_deflatexeps stores the EPS outside the procedure; new PSL_plotlatexepsdef places it. PSL_plotlatexeps works as before.
• gmt_plot.c: use them for LaTeX tags (replaces the /tmp workaround). The tag is on top of the panel, and +g, +p and +s work.
• Docs: removed the note saying LaTeX is not allowed in tags.

Note: use subplot set -A; subplot begin -A cannot take LaTeX because there each letter/digit means auto-numbering.

Script to test

gmt begin fig png
  gmt subplot begin 2x2 -Fs5c -A+jBR+o0.2c+gwhite+p1p,blue+s
    gmt subplot set 0,0 -A"@[\nabla^2@["
    gmt basemap -R0/10/0/10 -JX5c -Bafg
    gmt subplot set 0,1 -AB
    gmt basemap -R0/10/0/10 -JX5c -Bafg
    gmt subplot set 1,0 -A"@[\sum_i x_i@["
    gmt basemap -R0/10/0/10 -JX5c -Bafg -B+glightblue
    gmt subplot set 1,1 -A4
    gmt basemap -R0/10/0/10 -JX5c -Bafg -B+glightblue
  gmt subplot end
gmt end

[Esteban82]

BTW, I think that the WIP Check is failing.

[Esteban82]

BTW, I think that the WIP Check is failing.

Now it is working.

[joa-quim]

Looks a self contained change, and if it works.