item B: force HAVE_FFMPEG=TRUE on windows-MSVC (FFMPEG_FOUND=1 but doesn't stick)

Instrumented run proved: HAVE_FFMPEG=FALSE despite FFMPEG_FOUND=1 + avcodec 62.28/
avformat 62.12/avutil 60.26/swscale 9.5 all detected + FFMPEG_LIBRARIES/INCLUDE_DIRS
correct. OpenCV's HAVE_FFMPEG doesn't stick on windows-MSVC (scope/gate quirk). All
the vars cap_ffmpeg needs ARE present, so force HAVE_FFMPEG=TRUE in detect_ffmpeg.

Author: Sunrisepeak

.github/workflows/snapshot-windows-opencv.yml

@@ -65,6 +65,14 @@ jobs:
             echo 'message(STATUS "mcpp-dbg: HAVE_FFMPEG=[${HAVE_FFMPEG}] FFMPEG_FOUND=[${FFMPEG_FOUND}] HAVE_FFMPEG_WRAPPER=[${HAVE_FFMPEG_WRAPPER}]")'
             echo 'message(STATUS "mcpp-dbg: versions avcodec=[${FFMPEG_libavcodec_VERSION}] avformat=[${FFMPEG_libavformat_VERSION}] avutil=[${FFMPEG_libavutil_VERSION}] swscale=[${FFMPEG_libswscale_VERSION}]")'
             echo 'message(STATUS "mcpp-dbg: used=[${_used_ffmpeg_libraries}] FFMPEG_LIBRARIES=[${FFMPEG_LIBRARIES}]")'
+            # windows-MSVC: everything is detected (FFMPEG_FOUND=1 + all libs/versions/
+            # include-dirs) but OpenCV's HAVE_FFMPEG does not stick. Force it — the vars
+            # cap_ffmpeg needs are all correct. Set in this scope AND parent (detect_ffmpeg
+            # is include()d, so directory-scope set propagates; guard PARENT_SCOPE).
+            echo 'if(FFMPEG_FOUND AND NOT HAVE_FFMPEG)'
+            echo '  set(HAVE_FFMPEG TRUE)'
+            echo '  message(STATUS "mcpp: forced HAVE_FFMPEG=TRUE (FFMPEG_FOUND=1, all components detected on windows-MSVC)")'
+            echo 'endif()'
           } >> "$DF"
       - name: cmake configure (clang-cl, +videoio/FFmpeg via pkg-config, headless)
         shell: bash