input_stream: add method _cheap_seeking()

Move code from the Vorbis decoder plugin.

src/decoder/VorbisDecoderPlugin.cxx

@@ -139,9 +139,7 @@ vorbis_is_open(struct vorbis_input_stream *vis, OggVorbis_File *vf,
 {
 	vis->decoder = decoder;
 	vis->input_stream = input_stream;
-	vis->seekable = input_stream->seekable &&
-		(input_stream->uri == NULL ||
-		 !uri_has_scheme(input_stream->uri));
+	vis->seekable = input_stream_cheap_seeking(input_stream);
 
 	int ret = ov_open_callbacks(vis, vf, NULL, 0, vorbis_is_callbacks);
 	if (ret < 0) {

src/input_stream.c

@@ -22,6 +22,7 @@
 #include "input_registry.h"
 #include "input_plugin.h"
 #include "input/rewind_input_plugin.h"
+#include "uri.h"
 
 #include <glib.h>
 #include <assert.h>
@@ -114,6 +115,12 @@ input_stream_lock_wait_ready(struct input_stream *is)
 	g_mutex_unlock(is->mutex);
 }
 
+bool
+input_stream_cheap_seeking(const struct input_stream *is)
+{
+	return is->seekable && (is->uri == NULL || !uri_has_scheme(is->uri));
+}
+
 bool
 input_stream_seek(struct input_stream *is, goffset offset, int whence,
 		  GError **error_r)

src/input_stream.h

@@ -167,6 +167,13 @@ gcc_nonnull(1)
 void
 input_stream_lock_wait_ready(struct input_stream *is);
 
+/**
+ * Determines whether seeking is cheap.  This is true for local files.
+ */
+gcc_pure gcc_nonnull(1)
+bool
+input_stream_cheap_seeking(const struct input_stream *is);
+
 /**
  * Seeks to the specified position in the stream.  This will most
  * likely fail if the "seekable" flag is false.