Define scene constants alongside scene_scripts (#979)

Generic SCENE_DEFAULT and SCENE_FINISHED are legacy values

macros/scripts/maps.asm

@@ -10,13 +10,26 @@ DEF object_const_def EQUS "const_def 2"
 MACRO def_scene_scripts
 	REDEF _NUM_SCENE_SCRIPTS EQUS "_NUM_SCENE_SCRIPTS_\@"
 	db {_NUM_SCENE_SCRIPTS}
+	const_def
 	DEF {_NUM_SCENE_SCRIPTS} = 0
 ENDM
 
+MACRO scene_const
+;\1: scene id constant
+	const \1
+	EXPORT \1
+ENDM
+
 MACRO scene_script
 ;\1: script pointer
+;\2: scene id constant
 	dw \1
 	dw 0 ; filler
+	if _NARG == 2
+		scene_const \2
+	else
+		const_skip
+	endc
 	DEF {_NUM_SCENE_SCRIPTS} += 1
 ENDM