Name overworld tilemaps for their bg, add layer type constants

include/global.fieldmap.h

@@ -9,6 +9,12 @@
 #define METATILE_COLLISION_SHIFT 10
 #define METATILE_ELEVATION_MASK 0xF000
 
+enum {
+    METATILE_LAYER_TYPE_NORMAL,  // Metatile uses middle and top bg layers
+    METATILE_LAYER_TYPE_COVERED, // Metatile uses bottom and middle bg layers
+    METATILE_LAYER_TYPE_SPLIT,   // Metatile uses bottom and top bg layers
+};
+
 #define METATILE_ID(tileset, name) (METATILE_##tileset##_##name)
 
 // Rows of metatiles do not actually have a strict width.

include/overworld.h

@@ -40,20 +40,18 @@ struct LinkPlayerObjectEvent
     u8 movementMode;
 };
 
-// Exported RAM declarations
 extern struct WarpData gLastUsedWarp;
 extern struct LinkPlayerObjectEvent gLinkPlayerObjectEvents[4];
 
-extern u16 *gBGTilemapBuffers1;
-extern u16 *gBGTilemapBuffers2;
-extern u16 *gBGTilemapBuffers3;
+extern u16 *gOverworldTilemapBuffer_Bg2;
+extern u16 *gOverworldTilemapBuffer_Bg1;
+extern u16 *gOverworldTilemapBuffer_Bg3;
 extern u16 gHeldKeyCodeToSend;
 extern void (*gFieldCallback)(void);
 extern bool8 (*gFieldCallback2)(void);
 extern u8 gLocalLinkPlayerId;
 extern u8 gFieldLinkPlayerCount;
 
-// Exported ROM declarations
 extern const struct UCoords32 gDirectionToVectors[];
 
 void DoWhiteOut(void);