try to index media immediately on import

Since indexing is supplemental, it can be made a background task that can occur
at more or less any time. So we run it as soon as possible.

app/codec/decoder.h

@@ -219,6 +219,17 @@ public:
    */
   virtual void Conform(const AudioRenderingParams& params);
 
+  /**
+   * @brief Create an index for this media
+   *
+   * Indexes are used to improve speed and reliability of imported media. Calling Retrieve() will automatically check
+   * for an index and create one if it doesn't exist.
+   *
+   * Indexing is slow so it's recommended to do it in a background thread. Index() must be called while the Decoder is
+   * open, and does not automatically call Open() and Close() the Decoder. The caller must call thse manually.
+   */
+  virtual void Index();
+
 protected:
   bool open_;