Improved performance of EPUB novels with many images (Patreon)

Some novels have a lot of images in them, and when the novel is saved in EPUB format, they will be saved in the epub file.

Previously, the epub files generated by the downloader had performance issues. If the image size was large, some readers might freeze when opening the file. Now I have rewritten the relevant code to solve this problem (version 17.1.0).