ogg, ffmpeg: try to decode, even when the stream is not seekable

Ogg and ffmpeg detection was disabled when the stream was not
seekable, because the detection was too expensive.  Since the curl
input stream can now rewind the stream cheaply, we can re-enable
detection on streams.

src/decoder/ffmpeg_plugin.c

@@ -210,7 +210,7 @@ ffmpeg_helper(struct input_stream *input, bool (*callback)(BasePtrs *ptrs),
 static bool
 ffmpeg_try_decode(struct input_stream *input)
 {
-	return input->seekable && ffmpeg_helper(input, NULL, NULL);
+	return ffmpeg_helper(input, NULL, NULL);
 }
 
 static enum decoder_command