GUI: improve sample type conversion

src/engine/sample.h

@@ -269,6 +269,13 @@ struct DivSample {
    */
   bool resample(double sRate, double tRate, int filter);
 
+  /**
+   * convert sample depth.
+   * @warning do not attempt to do this outside of a synchronized block!
+   * @param newDepth the new depth.
+   */
+  void convert(DivSampleDepth newDepth);
+
   /**
    * initialize the rest of sample formats for this sample.
    */