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.
Mike-Solar
7a9e11b729