Track checkpoint levels

src/arena.h

@@ -21,6 +21,7 @@ struct Arena {
     U32 base_pos;     // absolute position of base relative to the first linked arena
     ArenaFlags flags;
     Arena *prev;
+    S32 checkpoint_level;
 };
 
 typedef struct ArenaCheckPoint ArenaCheckPoint;
@@ -45,3 +46,4 @@ void *arena_push(Arena *arena, U32 size, U32 align);
 ArenaCheckPoint arena_temp_begin(Arena *arena);
 void arena_temp_end(ArenaCheckPoint *checkpoint);
 void arena_pop_to(Arena *arena, U32 abs_pos);
+void arena_reset(Arena *arena);