meson: build the bundled ffmpeg without unused components

[CoffeeFlux]

ffms2 only demuxes and decodes local files, so the bundled ffmpeg doesn't need libavdevice, libavfilter, encoders, muxers or network support. This disables them through the meson port's own options, with no patches. It only affects builds that use the bundled ffmpeg (CI on Windows and macOS, and local fallback builds).

• ffmpeg compiles drop from 2,186 to 1,447, about a third fewer. Locally that was 29% less CPU, and a static ffms2 test binary went from 19.6 MB to 16.3 MB.
• Every decoder, parser and bitstream filter is still built. The only demuxers lost are network-streaming ones: RTP, RTSP, SAP and SDP.
• The file protocol, which ffms2 uses to open files, stays.

Testing (macOS):

• Built ffms2 against both versions and ran a small program that does what Aegisub does: index, decode video frames to BGRA, and decode audio. It covered H.264 (8- and 10-bit), HEVC, VP9, AV1, MPEG-2, MPEG-4 and ProRes in MP4, MKV, WebM, TS, AVI and MOV, plus FLAC, MP3 and WAV. Both versions gave identical results.
• Aegisub builds against the trimmed static ffmpeg and its tests pass.

🤖 Generated with Claude Code