feat(tiff): Support GPS fields, and other metadata enhancements

[lgritz]

Most visible change: The TIFF reader and writer now correctly handle GPS tags.

Fixes #5049

But along the way, and to get it completely right, a fairly extensive refactoring of our TIFF tag handling was needed. The libtiff behavior around different tags is extremely convoluted, and there were a bunch of cases we either didn't handle correctly, or were dropping on the floor. So there are also some other non-GPS tags that we missed all along but now read properly.

Note that the support for separate IFDs for GPS tags is only supported via the API in libtiff 4.2 and newer, so this is disabled when bulding against older libtiff versions. (Our support of old libtiff versions extends extraordinarily far back in time.)

[lgritz]

The failures on the wheel builds are unrelated to this PR and are happening in main. We are investigating that separately, and it should not inhibit review and acceptance of this PR.

[lgritz]

@kjellpeterson-shotover Are you able to tell if this patch satisfies your requirements as described in #5049 ?

[Copilot]

Pull request overview

Adds proper TIFF GPS IFD read/write support (when building against libtiff ≥ 4.2) and refactors TIFF tag handling to preserve more Exif/TIFF metadata that was previously ignored or dropped.

Changes:

• Write GPS tags into a dedicated GPS IFD (libtiff ≥ 4.2) and read them back via TIFFTAG_GPSIFD scanning.
• Generalize TIFF tag extraction to correctly handle more libtiff field passing conventions (including multi-value fields like YCbCrSubsampling and PageNumber).
• Extend tests: add a GPS-tagged TIFF fixture and update reference outputs to reflect newly preserved metadata.

Reviewed changes

Copilot reviewed 19 out of 20 changed files in this pull request and generated 5 comments.

Show a summary per file

File	Description
testsuite/tiff-suite/ref/out.txt	Updates golden output to include newly detected Exif/TIFF fields (e.g., ISO, PhotographicSensitivity, YCbCrSubsampling, PageNumber).
testsuite/tiff-suite/ref/out-jpeg9d-alt.txt	Same as above for jpeg9d-alt baseline.
testsuite/tiff-suite/ref/out-jpeg9b.txt	Same as above for jpeg9b baseline.
testsuite/tiff-suite/ref/out-alt2.txt	Same as above for alt2 baseline.
testsuite/tiff-suite/ref/out-alt.txt	Same as above for alt baseline.
testsuite/tiff-misc/src/gps.tif	Adds a GPS metadata fixture TIFF for round-trip testing.
testsuite/tiff-misc/run.py	Adds a GPS read/write test and info dump of the round-tripped output.
testsuite/tiff-misc/ref/out.txt	Updates baseline for older libtiff variants (no GPS IFD support).
testsuite/tiff-misc/ref/out-libtiff430.txt	Updates baseline for libtiff 4.3+ to include GPS tag output.
testsuite/tiff-misc/ref/out-libtiff410.txt	Updates baseline for libtiff 4.1 (no GPS IFD support).
testsuite/tiff-misc/ref/out-libtiff403.txt	Updates baseline for libtiff 4.0.3 (no GPS IFD support).
testsuite/tiff-misc/ref/out-libtiff403-c.txt	Updates baseline variant for libtiff 4.0.3-c (no GPS IFD support).
testsuite/tiff-misc/ref/out-libtiff403-b.txt	Updates baseline variant for libtiff 4.0.3-b (no GPS IFD support).
testsuite/tiff-misc/ref/out-libiff470.txt	Adds a new libtiff 4.7-specific baseline variant.
testsuite/misnamed-file/ref/out.txt	Updates golden output to include tiff:PageNumber.
src/tiff.imageio/tiffoutput.cpp	Refactors Exif writing to support GPS IFD writing and shared directory-writing helper.
src/tiff.imageio/tiffinput.cpp	Refactors tag reading to handle more libtiff calling conventions; adds GPS IFD scanning.
src/libOpenImageIO/exif.cpp	Fixes Exif:YCbCrSubsampling count and adds gps_tag_lookup implementation.
src/include/OpenImageIO/tiffutils.h	Exposes gps_tag_lookup in the public API.
src/cmake/externalpackages.cmake	Recommends libtiff ≥ 4.2 to unlock GPS IFD support.

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