Add RGBnoQUANT option to PS_COLOR_MODEL

[RyanHaotianPang]

Description of proposed changes

Adds PS_COLOR_MODEL=RGBnoQUANT, an opt-in compatibility setting for
third-party PostScript/PDF editors that incorrectly interpret low-bit-depth
indexed RGB raster images. It addresses #9171, where Adobe Illustrator may
change raster colors when opening or embedding a GMT-generated PDF.

RGBnoQUANT keeps the normal RGB color mode for all PostScript colors. Its
only difference is that 24-bit RGB raster images bypass the existing
indexed-color optimization and are emitted through the existing direct
/DeviceRGB output path. Default PS_COLOR_MODEL=RGB output, CPT
interpolation, and grdimage color calculations are unchanged.

The setting is parsed case-insensitively and gmt get PS_COLOR_MODEL reports
the canonical value rgbnoquant.

Fixes #9171

Visual comparison

Tests

• Added test/postscriptlight/rgbnoquant.sh with a four-color RGB PPM image.
  It verifies that normal RGB still emits /Indexed /DeviceRGB with reduced
  bits, while mixed-case RGBnoQUANT round-trips through gmt get, emits
  direct /DeviceRGB, and contains no indexed color space. It also checks
  that switching back to RGB restores indexed output.
• ctest --test-dir build --output-on-failure -R 'test/postscriptlight/rgbnoquant\\.sh' (1/1 passed).
• Ghostscript renders of the normal-RGB and RGBnoQUANT regression outputs were
  pixel-identical.
• Manually verified the modified PDF resolves the Illustrator color issue.

Reminders

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• If adding new functionality, add a detailed description to the documentation and/or an example.

[joa-quim]

Thanks for this PR, that looks good. I tested it locally and no side effects were detected.

[Esteban82]

Hi @RyanHaotianPang, thanks for the contribution! Did you use any AI assistant to help write this PR? If so, which one?

[RyanHaotianPang]

Hi, thanks! @Esteban82 I used OpenAI Codex to assist with the implementation and testing.