Build a Mesa llvmpipe with the gl_PrimitiveID fix for Ubuntu 26.04

[Fedr]

Adds the workflow and the README section for the Mesa asset already published as mesa-llvmpipe-77251a4-ubuntu26.

Why

Ubuntu 26.04 ships Mesa 26.0.8, which mis-reads gl_PrimitiveID in a fragment shader that also has user varyings and no geometry shader — mesa#15660. MeshInspector's picker is exactly that shape, so on the ubuntu26 CI leg it picked a face id of -1090332440 out of a 512-face mesh and crashed in MR::toTriPoint.

The fix reached Mesa main on 2026-07-22, seven days after the 26.2 branch point, and was never nominated for stable: it is contained in one branch and zero tags, so no Mesa release and no PPA has it — kisak-mesa for resolute is 26.1.7 and still affected. There is nothing to install and nothing to wait for, hence building it.

What

build-mesa-llvmpipe.yml, one job per arch, pushed-to-file trigger on any branch but main, Mesa commit as a dispatch input:

• Built in an ubuntu:26.04 container against Resolute's own llvm-21-dev (1:21.1.8 — the same LLVM its stock llvmpipe links), so glibc, libdrm, libglvnd and X11 match the image that will consume it. That is also why it is cheap: no LLVM build.
• llvmpipe only — Vulkan, video codecs and tools disabled. 25 MB installed, ~6 MB compressed, 6 min on x64 and 3 min on arm64.
• -Dglvnd=true, so it supplies libGLX_mesa.so.0, libEGL_mesa.so.0, libgbm, libglapi and the DRI modules while libglvnd stays the distro's. Selection is three env vars, which keeps it opt-in and trivially reversible.
• The smoke test runs glxinfo -B under xvfb through those env vars and fails the job if the version still reports Mesa 26.0. — a build that silently does not take effect cannot be published. Both arches reported 4.6 (Compatibility Profile) Mesa 26.3.0-devel (git-77251a488e).

Assets are named after the Mesa short commit, so the tag, the asset names and the README curl pattern all agree and a future rebuild needs no renaming. I verified the published tarball round-trips: download, sha256sum -c OK, and it contains libGLX_mesa.so.0, libEGL_mesa.so, dri/swrast_dri.so, libgbm.

Scope and retirement

This helps CI only. Users on llvmpipe stay affected until MeshLib takes the primitive id from somewhere else — MeshInspector/MeshLib#6774 tried and was closed, because the id it used is only valid in corner mode and desktop rendering uses vertex mode for crease-free meshes.

The README says to retire this once a Mesa >= 26.3 reaches the archive or kisak-mesa. The commit is in the tag deliberately: an image pinned by content checksum must not silently acquire a different driver — the same objection that ruled out pointing the image at a rolling PPA.

Nothing consumes it yet; wiring the ubuntu26 image is the next step.