ci: fix the vcpkg ffmpeg manifest feature and pin

vcpkg resolves the ffmpeg dependency before anything builds and rejects
the manifest because the pinned 8.1.2#3 port has no `png` feature
("ffmpeg@8.1.2#3 does not have required feature png needed by oak"),
so every desktop job died in "Install dependencies (vcpkg manifest)"
and never reached the build or test steps.

PNG decoding in FFmpeg needs zlib (png_decoder_deps=zlib); libpng is
only the encoder backend and this port never enables it. Request
`zlib` instead.

Also point the stale comments/docs at the actual pin: the override is
8.1.2#3 (matching the ffmpeg-next 8.x binding after the 9.0.0 binding
was found broken upstream), not 9.0.1#1.

docs/build.md

@@ -39,7 +39,7 @@ macOS, Linux, and Windows. For the Chinese version see
 - **CI/CD uses the vcpkg manifest instead of `build-ffmpeg.sh`.** Every
   desktop job runs `vcpkg install --triplet <x64-windows | x64-linux |
   arm64-osx | arm64-linux>` against the repo-root `vcpkg.json`, which
-  pins FFmpeg to **9.0.1#1** (`overrides`) and every other port through
+  pins FFmpeg to **8.1.2#3** (`overrides`) and every other port through
   `builtin-baseline`; the tree lands in `vcpkg_installed/<triplet>` and
   jobs point `FFMPEG_DIR`/`PKG_CONFIG_PATH` at it. The C/C++ libraries
   outside that set (OCIO is vendored, the rest come from the system
@@ -97,7 +97,7 @@ cargo test  --workspace         # Linux: see "headless tests" below
 
 > **CI/CD note**: the GitHub Windows CI/CD no longer uses this path — it
 > builds MSVC-ABI on `warp-windows-2025-vs2026-x64-16x` with vcpkg
-> manifest mode (`vcpkg.json` at the repo root: FFmpeg 9.0.1 with every
+> manifest mode (`vcpkg.json` at the repo root: FFmpeg 8.1.2 with every
 > free codec + hwaccel, pkgconf, librsvg, all pinned by
 > `overrides`/`builtin-baseline`) and the vendored static OCIO. The
 > MSYS2 flow below remains the documented local-build alternative.