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

src/decoder/oggflac_plugin.c

@@ -281,11 +281,6 @@ static struct tag *oggflac_TagDup(const char *file)
 
 static bool oggflac_try_decode(struct input_stream *inStream)
 {
-	if (!inStream->seekable)
-		/* we cannot seek after the detection, so don't bother
-		   checking */
-		return true;
-
 	return ogg_stream_type_detect(inStream) == FLAC;
 }
 

src/decoder/oggvorbis_plugin.c

@@ -364,11 +364,6 @@ static struct tag *oggvorbis_TagDup(const char *file)
 static bool
 oggvorbis_try_decode(struct input_stream *inStream)
 {
-	if (!inStream->seekable)
-		/* we cannot seek after the detection, so don't bother
-		   checking */
-		return true;
-
 	return ogg_stream_type_detect(inStream) == VORBIS;
 }