track arena free size when allocating blocks

src/util.h

@@ -25,6 +25,7 @@ u64 utilFNV64a_str(const char* str, u64 hval = FNV1_64_INIT);
 struct MemoryArena
 {
 	size_t max_size;
+	size_t free_size;
 	void* head;
 	void* next_free;
 };
@@ -117,6 +118,7 @@ arenaAllocateBlock(MemoryArena* arena, size_t block_size)
 	assert(block_size > 0);
 	void* ret = arena->next_free;
 	arena->next_free = (uint8_t*) arena->next_free + block_size;
+	arena->free_size = arenaGetFreeSize(arena);
 	return ret;
 }