factor out a cutscene layer formula

include/z64cutscene.h

@@ -346,7 +346,7 @@ typedef enum CutsceneDestination {
 
 // the primary purpose of these values is to select `gSaveContext.sceneLayer`
 //      CS_INDEX_AUTO:     [SCENE_LAYER_CHILD_DAY .. SCENE_LAYER_ADULT_NIGHT]
-//      CS_INDEX_[0 .. A]: SCENE_LAYER_CUTSCENE_FIRST + (cutscene index & 0xF)
+//      CS_INDEX_[0 .. A]: GET_CUTSCENE_LAYER(index)
 // `z_demo.c` is the main user
 #define CS_INDEX_AUTO 0x0000
 #define CS_INDEX_0 0xFFF0

include/z64save.h

@@ -392,6 +392,7 @@ typedef enum SceneLayer {
 } SceneLayer;
 
 #define IS_CUTSCENE_LAYER (gSaveContext.sceneLayer >= SCENE_LAYER_CUTSCENE_FIRST)
+#define GET_CUTSCENE_LAYER(index) (SCENE_LAYER_CUTSCENE_FIRST + (index & 0xF))
 
 typedef enum LinkAge {
     /* 0 */ LINK_AGE_ADULT,