YM2612: implement 9xxx, part 1

does not work on VGM export yet

src/engine/dispatch.h

@@ -288,6 +288,9 @@ struct DivRegWrite {
    *   - x is the instance ID
    * - 0xffffxx04: switch sample bank
    *   - for use in VGM export
+   * - 0xffffxx05: set sample position
+   *   - xx is the instance ID
+   *   - data is the sample position
    * - 0xffffffff: reset
    */
   unsigned int addr;