Use prepend.js to fix worker script loading issue

Author: jeromewu

wasm/build-scripts/build-ffmpeg.sh

@@ -12,6 +12,7 @@ FLAGS=(
   -lavdevice -lavfilter -lavformat -lavcodec -lswresample -lswscale -lavutil -lpostproc -lm -lx264 -lx265 -lvpx -lwavpack -lmp3lame -lfdk-aac -lvorbis -lvorbisenc -lvorbisfile -logg -ltheora -ltheoraenc -ltheoradec -lz -pthread
   fftools/ffmpeg_opt.c fftools/ffmpeg_filter.c fftools/ffmpeg_hw.c fftools/cmdutils.c fftools/ffmpeg.c
   -o wasm/dist/ffmpeg-core.js
+  --pre-js wasm/src/prepend.js                  # fix worker script issue
   -s USE_SDL=2                                  # use SDL2
   -s USE_PTHREADS=1                             # enable pthreads support
   -s PROXY_TO_PTHREAD=1                         # detach main() from browser/UI main thread

wasm/src/prepend.js

@@ -0,0 +1,8 @@
+Module['locateFile'] = function(path, prefix) {
+  if (typeof window !== 'undefined'
+    && typeof window.FFMPEG_CORE_WORKER_SCRIPT !== 'undefined'
+    && path.endsWith('ffmpeg-core.worker.js')) {
+    return window.FFMPEG_CORE_WORKER_SCRIPT;
+  }
+  return prefix + path;
+}