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.