load a tile from cache or tile server

def load_tile(self, tile):
		'''load a tile from cache or tile server'''

		# see if its in the tile cache
		key = tile.key()
		if key in self._tile_cache:
			img = self._tile_cache[key]
			if img == self._unavailable:
				img = self.load_tile_lowres(tile)
				if img is None:
					img = self._unavailable
				return img


		path = self.tile_to_path(tile)
		try:
			ret = cv.LoadImage(path)
			# if it is an old tile, then try to refresh
			if os.path.getmtime(path) + self.refresh_age < time.time():
				try:
					self._download_pending[key].refresh_time()
				except Exception:
					self._download_pending[key] = tile
					self.start_download_thread()
			# add it to the tile cache
			self._tile_cache[key] = ret
			while len(self._tile_cache) > self.cache_size:
				self._tile_cache.popitem(0)
			return ret
		except IOError as e:
			# windows gives errno 0 for some versions of python, treat that as ENOENT
			# and try a download
			if not e.errno in [errno.ENOENT,0]:
				raise
			pass
		if not self.download:
			img = self.load_tile_lowres(tile)
			if img is None:
				img = self._unavailable
			return img

		try:
			self._download_pending[key].refresh_time()
		except Exception:
			self._download_pending[key] = tile
		self.start_download_thread()

		img = self.load_tile_lowres(tile)
		if img is None:
			img = self._loading
		return img