improve: ffmpeg, use 4 threads for less errors

s/context/controllers/video-export/controller.ts

@@ -97,7 +97,7 @@ export class VideoExport {
 	async #extract_frames_from_video(effect: VideoEffect) {
 		const file_result = await fetchFile(effect.file)
 		await this.ffmpeg.ffmpeg.writeFile(effect.file.name, file_result)
-		await this.ffmpeg.ffmpeg.exec(["-i", effect.file.name, `out${effect.id}_%d.png`])
+		await this.ffmpeg.ffmpeg.exec(["-threads", "4","-i", effect.file.name, `out${effect.id}_%d.png`])
 		this.decoded_effects.set(effect.id, effect.id)
 	}
 

s/context/controllers/video-export/helpers/FFmpegHelper/helper.ts

@@ -46,15 +46,13 @@ export class FFmpegHelper {
 		await this.ffmpeg.createDir("/thumbnails")
 		await this.ffmpeg.createDir("/segments")
 		await this.ffmpeg.writeFile(file.name, result)
-		// remux to mkv container, which supports more variety of codecs
-		await this.ffmpeg.exec(["-i", file.name, "-c", "copy", "container.mkv"])
 		// split into 5 second segments, so user can get new filmstrips every 5 seconds
-		await this.ffmpeg.exec(["-i", "container.mkv", "-c", "copy", "-map", "0", "-reset_timestamps", "1", "-f", "segment", "-segment_time", "5", "segments/out%d.mkv"])
+		await this.ffmpeg.exec(["-threads", "4","-i", file.name, "-c", "copy", "-map", "0", "-reset_timestamps", "1", "-f", "segment", "-segment_time", "5", "segments/out%d.mp4"])
 		const segments = await this.ffmpeg.listDir("/segments")
 
 		for(const segment of segments) {
 			if(!segment.isDir) {
-				await this.ffmpeg.exec(["-i", `segments/${segment.name}`, "-filter_complex", `select='not(mod(n\,1))',scale=100:50`, "-an", "-c:v", "libwebp", `thumbnails/${segment_number}_out%d.webp`])
+				await this.ffmpeg.exec(["-threads", "4","-i", `segments/${segment.name}`, "-filter_complex", `scale=100:50`, "-an", "-c:v", "libwebp", "-preset","icon" ,`thumbnails/${segment_number}_out%d.webp`])
 				const frames = await this.ffmpeg.listDir("/thumbnails")
 				for(const frame of frames) {
 					if(!frame.isDir) {