SotA:17: Use screen_fade for sacrifice fade-out

There is a problem that the skeleton reverts to the base frame
during the fade. Previously the skeleton was deleted so this was
not evident.

data/campaigns/Secrets_of_the_Ancients/scenarios/17_Mortality.cfg

@@ -322,11 +322,18 @@ Not far inside the cave was a ruined castle built in a style I did not recognize
             flag=sacrifice
         [/animate_unit]
 
-        [hide_unit]
-            # All units.
-        [/hide_unit]
+        # TODO: The skeleton should keep the final animation frame here,
+        # rather than flipping back to the standing animation for the fade.
 
-        {FADE_TO_BLACK_HOLD 1000}
+        [screen_fade]
+            red,green,blue=0,0,0
+            alpha=255
+            duration=1000
+        [/screen_fade]
+
+        [delay]
+            time=2000
+        [/delay]
 
         [fire_event]
             name=lichify  # See: sota-utils.cfg

data/campaigns/Secrets_of_the_Ancients/units/Skele_Sacrifice.cfg

@@ -22,7 +22,7 @@
             layer=100
         [/frame]
         [frame]
-            image="units/undead/skeleton-se-sacrifice7.png:1000"
+            image="units/undead/skeleton-se-sacrifice7.png:500"
             x=33
             y=0
             layer=100