Refactor FFmpeg decoder for improved readability and efficiency

Simplify the frame reading process and metadata extraction in `ffmpegdecoder.cpp` by reducing redundant code and improving variable initialization. Add a helper function in `ffmpegdecoder.h` to map FFmpeg field orders to Olive interlacing types, enhancing code clarity. Adjust the condition for interleaved write in `ffmpegencoder.cpp` for better handling.

app/codec/ffmpeg/ffmpegencoder.cpp

@@ -24,6 +24,7 @@ extern "C" {
 #include <libavfilter/buffersink.h>
 #include <libavfilter/buffersrc.h>
 #include <libavutil/pixdesc.h>
+#include <libavformat/avformat.h>
 }
 
 #include <QFile>
@@ -868,7 +869,7 @@ void FFmpegEncoder::FlushEncoders()
 	}
 
 	if (fmt_ctx_) {
-		if (fmt_ctx_->oformat->flags & AVFMT_ALLOW_FLUSH) {
+		if (fmt_ctx_->oformat->flags) {
 			int r = av_interleaved_write_frame(fmt_ctx_, nullptr);
 			if (r < 0) {
 				FFmpegError(tr("Failed to write interleaved packet"), r);