Use constants for VBlank handlers

engine/movie/credits.asm

@@ -76,7 +76,7 @@ Credits::
 ; BUG: Credits sequence changes move selection menu behavior (see docs/bugs_and_glitches.md)
 	ldh a, [hVBlank]
 	push af
-	ld a, $5
+	ld a, VBLANK_CREDITS
 	ldh [hVBlank], a
 	ld a, TRUE
 	ldh [hInMenu], a

engine/movie/intro.asm

@@ -45,9 +45,10 @@ CrystalIntro:
 	ret
 
 .InitRAMAddrs:
+	assert VBLANK_NORMAL == 0
 	xor a
 	ldh [hVBlank], a
-	ld a, $1
+	ld a, TRUE
 	ldh [hInMenu], a
 	xor a
 	ldh [hMapAnims], a