Fix Brotli, skcms, and x264 for FFmpeg (#10270)

Fixes three dependency failures encountered while enabling FFmpeg's
optional codecs in #10256, without changing published module versions:

- `brotli@1.2.0.bcr.3` makes the 16-bit rotation and hash-counter
conversions explicit so GCC 8 accepts the existing strict warnings. It
removes the presubmit conversion-warning suppressions and adds an
exhaustive rotation test.
- `skcms@20250916.0.bcr.2` restores the per-element vector conversion
for GCC before 9, where `__builtin_convertvector` is unavailable. Modern
GCC and Clang keep the builtin. A public `skcms_Transform` test covers
byte/float conversion, rounding, clamping, and partial SIMD batches;
Debian 10 and Rocky Linux 8 join the existing platforms.
- `x264@2023.10.1.bcr.3` downloads the same commit from the GitHub
mirror after the VideoLAN URL returned inconsistent checksums in CI. The
replacement archive matches the existing published SHA256 exactly. Its
build targets and presubmit are unchanged.

Local validation passes: GCC 8.5 builds all Brotli sources with strict
warnings and passes the rotation test and CLI round trip; GCC 8.5 builds
all skcms SIMD implementations and passes the transform test in
fastbuild, opt, and dbg. Bazel 8.5 passes all 57 FFmpeg/dependency tests
on macOS ARM64 with Clang 21, including FFmpeg's 52 tests with optional
codecs enabled, the two new regression tests, and JPEG XL's three
existing tests. Presubmit retains the existing build targets and
configurations and now covers 78 configurations across the three
modules.

CI passes all 78 configurations, including the Bazel 7.x/8.x
incompatible-flag reruns. Registry validation, formatting, tooling
checks, and CLA checks also pass. [Buildkite
results](https://buildkite.com/bazel/bcr-presubmit/builds/40204).

The three modules retain their existing archive checksums. Brotli and
skcms also retain their existing source URLs; x264 changes only the
download host. These GitHub archives use BCR's documented URL-stability
exception; archive integrity and source-repository checks remain
enabled.

-zbarskybot
25 files changed
tree: 77e120df71ad34b81ff9c9c2e8d706db1b92688d
  1. .agent/
  2. .bazelci/
  3. .gemini/
  4. .github/
  5. docs/
  6. modules/
  7. tools/
  8. .bazelignore
  9. .bazelversion
  10. .editorconfig
  11. .git-blame-ignore-revs
  12. .gitattributes
  13. .gitignore
  14. .pre-commit-config.yaml
  15. .ruff.toml
  16. AUTHORS
  17. bazel_registry.json
  18. BUILD
  19. CODEOWNERS
  20. GEMINI.md
  21. incompatible_flags.yml
  22. LICENSE
  23. metadata.schema.json
  24. MODULE.bazel
  25. README.md
  26. WORKSPACE
README.md

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