Switch to flutter_quick_video_encoder?

https://pub.dev/packages/flutter_quick_video_encoder

will be much faster than creating lots of PNGs

e.g.

```
await FlutterQuickVideoEncoder.setup(
width: 1920,
height: 1080,
fps: 60,
videoBitrate: 2500000,
audioChannels: 1,
audioBitrate: 64000,
sampleRate: 44100,
filepath: "/documents/video.mp4",
);
for(int i = 0; i < frameCount; i++) {
Uint8List rgba = _renderVideoFrame(i); // your video function
Uint8List pcm = _renderAudioFrame(i); // your audio function
await FlutterQuickVideoEncoder.appendVideoFrame(rgba);
await FlutterQuickVideoEncoder.appendAudioFrame(pcm);
}
await FlutterQuickVideoEncoder.finish();
```