text: Add a new modifier to -D (-D+f<file>)

[joa-quim]

• Add new modifier +f to the -D option of text/pstext
• The file contains one (dx, dy) pair per text record, allowing each label to have its own displacement instead of a single fixed offset
• File reading uses GMT_Read_Data so the feature works transparently with GMT.jl in-memory datasets (virtual files)
• Values in the file are in the current length unit (PROJ_LENGTH_UNIT)
• Added shell test test/pstext/var_offset.sh

Example

  # offsets.txt has one dx,dy per label
  gmt text -R... -J... -F+f12p+jCM -D0+foffsets.txt << EOF
  -3  -3  SW corner
   3   3  NE corner
  EOF

Mostly written by Claude.

[github-actions]

Summary of changed images

This is an auto-generated report of images that have changed on the DVC remote

Status	Path
added	test/baseline/pstext/var_offset.ps
modified	test/baseline/pstext/

Image diff(s)

Details

Added images

• test/baseline/pstext/var_offset.png

Modified images

Path	Old	New

Report last updated at commit ef6fc0b

[remkos]

Looks good to me. I see that en passant you fixed a few other things.

[joa-quim]

OK I'll merge it, but I will need that some change the mode of the var_offset.sh script.

[Esteban82]

Just for the record, I run the test and looks good to me.

[seisman]

PR titles would be more informative if they adopted a format such as

module: description of the change.