Expand x265 CI to macOS/AArch64/Windows with parallel multilib builds and binary-reuse smoke tests

[Akilan-Sivakumar]

Summary

Expands the x265 CI pipeline from Linux-only to four platforms (Linux, macOS, AArch64 cross+QEMU, Windows), consolidates the per-suite test jobs into a per-platform test matrix, and cuts wall-clock time through build caching, parallelized multilib linking, and smoke-test binary reuse.

What changed

Platform coverage

• Added macOS (native, Apple Silicon), AArch64 (cross-compiled + QEMU user-mode), and Windows (MSVC) build + test jobs, matching the existing Linux job's matrix shape (8-bit / 10-bit / 12-bit / multilib).
• Consolidated the seven separate per-suite test jobs (test-cli-presets, test-rate-control, …) into one job per platform with a category matrix (presets-tune, rate-control, encoding-features, analysis-reuse, real-world, threading, hbd).

Build matrix

• Added Debug builds alongside the existing Release builds, as a new config: [Release, Debug] matrix dimension on Linux and Windows.
• Parallelized multilib linking: the 12-bit/10-bit components now build as extra parallel legs of the main build matrix (build-linux/build-windows); a thin build-linux-multilib/build-windows-multilib job downloads both component libs and does only the final 8-bit link step. Previously all three steps ran sequentially in one job.

Caching

• ccache/sccache keys are now scoped by PR number (in addition to branch/variant), preventing cross-PR cache poisoning under pull_request_target, whose cache scope is otherwise tied only to the base branch.

Bug fixes

• Windows builds were intermittently crashing at CMake-configure time because the checkout didn't fetch git tags, and x265's Windows version-string logic needs a tag present to embed into the resource file. Fixed by fetching full git history and tags during checkout.
• The Windows multilib build was uploading the wrong shared-library filename (x265.dll instead of the correct libx265.dll), so jobs that consumed that artifact couldn't find the library. Fixed the artifact path.
• macOS builds were printing Homebrew warnings about an untrusted third-party tap (aws/tap) on every run. Fixed by untapping it before installing dependencies.
• sccache is now pinned to v0.17.0 and downloaded directly from GitHub Releases instead of querying api.github.com/.../releases/latest, removing a flaky dependency on GitHub's unauthenticated API rate limit (60 req/hr, shared across all Actions runners globally).

Smoke tests

• smoke-linux/smoke-windows now reuse the CI's own prebuilt binaries via smoke-test.py --no-make --no-bench instead of having the harness rebuild from source

AArch64 QEMU tuning

• Trimmed the heaviest real-world test case's frame count (150 → 60) to reduce QEMU emulation time without dropping preset coverage.

Cleanup

• Removed unused "download multilib binary" steps from all four test-* jobs (binary was fetched but never invoked in any test command).
• Simplified needs: graphs to only what's functionally required: smoke-* depends on the multilib link job only (base build is covered transitively); summary-report depends only on the test-* jobs it actually consumes artifacts from.

[galad87]

A bit unrelated, but not too unrelated, still a macOS aarch64 cross-compiling issue, and my message over there was ignored: can #877 be merged?