switch from ST to MT

**Describe the bug**
When reloading from ST to MT, I can't reuse the (detached) ArrayBuffer which has been assigned as input data in the ST instance of FFmpeg

**To Reproduce**
```js
async function makeConfig(mt) {
return ({
coreURL: await window.ffmpegUtil.toBlobURL(`${baseURL}/ffmpeg-core.js`, 'text/javascript'),
wasmURL: await window.ffmpegUtil.toBlobURL(`${baseURL}/ffmpeg-core.wasm`, 'application/wasm'),
workerURL = mt ? await window.ffmpegUtil.toBlobURL(`${baseURL}/ffmpeg-core.worker.js`, 'text/javascript') : null
});
}

new FFmpeg();
await ffmpeg.load(await makeConfig(false));
await ffmpeg.writeFile("foo", new Uint8Array(data));
await ffmpeg.exec(['-i', "foo"]);

await ffmpeg.load(await makeConfig(true));
await ffmpeg.writeFile("foo", new Uint8Array(data)); // <-- TypeError: Cannot perform Construct on a detached ArrayBuffer
await ffmpeg.exec(['-i', "foo"]);
```

**Expected behavior**
I should be able to switch from ST to MT and reuse the data.

**Desktop (please complete the following information):**
- OS: Ubuntu
- Browser: Firefox 141 / Chromium 142

**Additional context**
After using `ffprobe` I know which codec the video is using. Only then can I choose whether MT is suitable. (For some codecs, like HEVC, it fails and thus must be avoided, but for all others it gives significant performances boost and is desirable)