export stringToUTF8() for Unicode characters (ffmpegwasm#12)

* export stringToUTF8() for Unicode characters

* export lengthBytesUTF8() for Unicode characters

Co-authored-by: jeromewu <jeromewus@gmail.com>

Author: cs8425

wasm/build-scripts/build-ffmpeg.sh

@@ -29,7 +29,7 @@ FLAGS=(
   -s MODULARIZE=1                               # use modularized version to be more flexible
   -s EXPORT_NAME="createFFmpegCore"             # assign export name for browser
   -s EXPORTED_FUNCTIONS="[_main]"  # export main and proxy_main funcs
-  -s EXPORTED_RUNTIME_METHODS="[FS, cwrap, ccall, setValue, writeAsciiToMemory]"   # export preamble funcs
+  -s EXPORTED_RUNTIME_METHODS="[FS, cwrap, ccall, setValue, writeAsciiToMemory, lengthBytesUTF8, stringToUTF8, UTF8ToString]"   # export preamble funcs
   -s INITIAL_MEMORY=2146435072                  # 64 KB * 1024 * 16 * 2047 = 2146435072 bytes ~= 2 GB
   --pre-js wasm/src/pre.js
   --post-js wasm/src/post.js