Add 8-up support to ipptransform

[Mr-Tao]

Summary

• add an explicit 2-by-4 layout for number-up=8 in ipptransform
• document 8 as a supported number-up value
• add direct transform and virtual IPP Everywhere regression coverage

Motivation

This is the libcups companion to OpenPrinting/cups#1672. Eight-up printing is particularly useful with ISO A-series paper, where the page aspect ratio makes this imposition geometrically natural. The current libcupsfilters PDF transform already supports an explicit 8-up layout.

The new end-to-end test uses ippeveprinter, submits the existing eight-page PDF fixture with number-up=8, verifies that the job retains the attribute and completes successfully, and checks that ipptransform places all eight input pages on one output page. No physical printer is required.

Testing

• ./configure --enable-debug
• make
• make -C tools test — existing suite: 202 tests, 79 passed, 0 failed, 123 skipped; new 8-up suite: 3 passed, 0 failed
• negative control with the new case 8 temporarily removed: the regression test fails on the former 1-up fallback as expected

Related UI changes:

• GTK 4: https://gitlab.gnome.org/GNOME/gtk/-/merge_requests/10258
• GTK 3: https://gitlab.gnome.org/GNOME/gtk/-/merge_requests/10259

Assisted-by: OpenAI Codex:GPT-5 [Codex]

[michaelrsweet]

I'm really not sure how I feel about adding this at a time when things like N-up are barely used... Will consider it but certainly not before libcups 3.1.