codec: cap FFmpeg decoder frame threads at 4

threads=auto spawns one decode thread per logical core per decoder (16
observed on a 24-core box); with cores-2 render workers decoding
concurrently the machine is oversubscribed several times over and the
playback-audio render thread gets starved (late chunks are dropped).
4 threads decode 1080p h264 far past real-time; preview throughput comes
from the worker pool, not per-decoder threading. Applies to both the
software open path and the hwaccel open path.

crates/oak-codec/src/hwdecode.rs

@@ -195,7 +195,7 @@ pub fn open_hw_accel(
 	// context frees it with the context.
 	unsafe { (*context.as_mut_ptr()).hw_device_ctx = device };
 	let mut opts = Dictionary::new();
-	opts.set("threads", "auto");
+	opts.set("threads", &crate::ffmpeg::decoder_threads());
 	context
 		.decoder()
 		.open_as_with(codec, opts)