Greatly reduce volume-change zipper artifacts with SW volume.

Uses a cosine factor to smoothly shift the PCM level from the old level
to the new one over the length of a frame.

Implements indirect calls to PCM scaling function instead of testing
conditions on every callback, cleanly assigning a different call to
do the volume transition. The volume change call then assigns the final
scaling function.

Change-Id: If1004b92a91c5ca766dd0e4014ec274636e8ed26
Reviewed-on: http://gerrit.rockbox.org/763
Reviewed-by: Michael Sevakis <jethead71@rockbox.org>
Tested: Michael Sevakis <jethead71@rockbox.org>

firmware/pcm.c

@@ -111,6 +111,10 @@ void pcm_play_stop_int(void);
  ** pcm_sw_volume.c **/
 static inline void pcm_play_dma_start_int(const void *addr, size_t size)
 {
+#ifdef HAVE_SW_VOLUME_CONTROL
+    /* Smoothed transition might not have happened so sync now */
+    pcm_sync_pcm_factors();
+#endif
     pcm_play_dma_start(addr, size);
 }