Fix encounter silhouette slide speed and blackout (#577)

The battle intro slide ran at 4px/frame over 40 frames -- twice the
original speed -- and drew the pics in their normal palette instead of
as black silhouettes.

SlidePlayerAndEnemySilhouettesOnScreen scrolls SCX from $90 to 0 at
2px/frame (~72 frames) under the %11100100 silhouette palette, only
running SET_PAL_BATTLE once the pics land. Match that: introSlide starts
at 80 with a 2px/frame offset (80 frames over the full 160px width), and
picImage bakes both pics through PAL_BLACK while introSlide > 0, exactly
like the evolution movie (#279).

parity_battle_intro_chrome waited a hardcoded 45 frames for the slide to
land; bumped to 85 to cover the slower slide.

src/battle/WideBattle.lua

@@ -320,7 +320,7 @@ function WideBattle.draw(battle)
   if sx == 0 and sy == 0 and fx and fx.shake and fx.shake > 0 then
     sx = battle.frame % 4 < 2 and 2 or -2
   end
-  local slide = (battle.introSlide or 0) * 4
+  local slide = (battle.introSlide or 0) * 2
 
   -- Each side keeps its original sprite pixels and placement math: the two
   -- 160x144 OAM regions are translated apart and clipped into the wider