Optimisation to dragging of volume slider - reduces flicker.

Retagged as 'CDFSFiler-2_43'

s/CDFSFiler

@@ -250,6 +250,7 @@ sharewindrive   #       4              ; the disc name block of the share window
 volumewindrive  #       4              ; the disc name block of the volume window
 
 dragging        #       4              ; flag to say whether we're dragging the volume bar
+draggingvolume  #       4              ; for optimisation when dragging
 
 ; ---------------------------
 cd_namedisc     #       4    ; for indirect data
@@ -1106,6 +1107,8 @@ event_volume_window_click
       BEQ       event_volume_cancel
 
       TEQ       r4, # VOLUME__DRAW_BAR
+      MOVEQ     r14, #&80000000
+      STREQ     r14, draggingvolume
       BEQ       event_volume_bar
 
       EXIT
@@ -1233,6 +1236,11 @@ event_volume_bar
       MOVGE     r4, # &ffffffff
       MOVGE     r4, r4, LSR # 16
 
+      LDR       r14, draggingvolume
+      TEQ       r4, r14
+      EXIT      EQ
+      STR       r4, draggingvolume
+
       LDR       r0, CurrentVolumeDrive
       MOV       r1, r4
       BL        set_volume_level