Fix font color transparency (red@50 in -F+f) being silently ignored

[Esteban82]

Done with Claude Sonnet 5
Cause: in PSL_setcolor(), font colors are internally treated as PSL_IS_STROKE, so embedded @<transparency> was applied to PEN_TRANSP instead of FILL_TRANSP — but text is painted as a filled shape, so it had no visible effect.
Fix: track that the color came from PSL_IS_FONT and route its transparency to FILL_TRANSP.

Without this fix, the "GMT" in the center has no transparency.

Full script:

gmt begin map png
    gmt coast -Rg -JH10c -Baf -Glightgreen -Slightblue
    echo GMT | gmt text -F+cMC+f40p,red@50      # Now works
    echo GMT | gmt text -F+cML+f40p,red -t50    # Works
    echo 180 40 | gmt plot -Sc1c -Gblack@50     # Works
gmt end

Fixes #9125

[seisman]

I guess the bug was introduced in #9046 or #8882.

[remkos]

I tested out the patch on a similar script for myself.
It works. Thanks!