Restore SDL2 + libopenal (avdevice deps); Redist 7.1.3

[oysteinkrog]

What this does

Restores SDL2.dll and libopenal-1.dll to the FFmpeg.AutoGen.Redist native payload and ships it as version 7.1.3. ffplay.exe stays dropped.

Why

The 7.1.2 slim repackaging removed ffplay.exe, SDL2.dll, and libopenal-1.dll on the premise that SDL2 + OpenAL were consumed only by ffplay.exe. That premise is wrong.

avdevice-if-61.dll links against SDL2.dll and libopenal-1.dll, and avdevice-if-61.dll is a static import of both ffmpeg.exe and ffprobe.exe. The Windows loader therefore resolves avdevice at process startup, which needs SDL2 + OpenAL; with them removed and absent from System32, every spawned ffmpeg/ffprobe invocation fails immediately with STATUS_DLL_NOT_FOUND (0xC0000135) before doing any work. 7.1.1 was immune only because its exes were statically linked (134 MB) and imported no av*-if DLLs. In-process FFmpeg.AutoGen bindings are unaffected because they never load avdevice.

objdump evidence on the 7.1.2 payload:

ffprobe.exe imports: avcodec-if-61 avdevice-if-61 avformat-if-61 avutil-if-59 swresample-if-5 swscale-if-8 avfilter-if-10 (+ KERNEL32/msvcrt/SHELL32)
ffmpeg.exe  imports: same set, including avdevice-if-61.dll
avdevice-if-61.dll imports: SDL2.dll  libopenal-1.dll  libcaca-0 libxml2-2 avcodec/avfilter/avformat/avutil-if + GDI32/ole32/...

A recursive dependency walk of the built output found exactly two DLLs missing from the whole graph: SDL2.dll and libopenal-1.dll. Adding them back flips the downstream MotionCatalyst FFmpeg test suite from 41 failed to 0.

What's in here

• SDL2.dll (2.4 MB) and libopenal-1.dll (1.9 MB) restored into FFmpeg/bin/, extracted from the committed mingw-w64-x86_64-ffmpeg-if-bundled-7.1-1-any.pkg.tar.zst; byte-identical to the copies already present in FFmpeg/bin/x64/.
• The SDL2.dll/libopenal-1.dll Exclude removed from both pack/copy ItemGroups in FFmpeg.AutoGen.Redist.csproj. ffplay.exe remains dropped (it is the one binary nothing else imports).
• FFmpeg.AutoGen.Redist version bumped to 7.1.3.
• verify_redist.py: forbidden set is now ffplay.exe + *.pdb only; the pe-imports gate additionally requires avdevice-if-61.dll in the exe import tables and still forbids a direct SDL2.dll import (which the exes correctly do not have). redist-manifest.txt regenerated (71 to 73 entries). Self-tests updated.

Net native payload is about 86 MB versus the 338 MB original 7.1.1, so the size win of the original slimming is preserved.

The genuinely correct long-term slim is to rebuild the IF bundle with avdevice disabled (or without its SDL/OpenAL device backends), letting the exes drop the avdevice import and shed SDL2 + OpenAL for real. That needs an FFmpeg rebuild and is out of scope here.

How to review

• python scripts/test_verify_redist.py — 19 tests OK.
• dotnet build FFmpeg.AutoGen.Redist -c Release && dotnet pack FFmpeg.AutoGen.Redist -c Release --no-build -o artifacts/nupkgs, then python scripts/verify_redist.py artifacts/nupkgs/FFmpeg.AutoGen.Redist.7.1.3.nupkg --expected-version 7.1.3 — all six gates PASS.

Fixes: DESKTOP-12084