remove auto live previews format option, fix slow PNG generation

        image = shared.state.current_image

        if image is not None:

            buffered = io.BytesIO()

            format = opts.live_previews_format

            save_kwargs = {}

            if format == "auto":

                if max(*image.size) > 256:

                    format = "jpeg"

            if opts.live_previews_image_format == "png":

                # using optimize for large images takes an enormous amount of time

                save_kwargs = {"optimize": max(*image.size) > 256}

            else:

                    format = "png"

                    save_kwargs = {"optimize": True}

            image.save(buffered, format=format, **save_kwargs)

                save_kwargs = {}

            image.save(buffered, format=opts.live_previews_image_format, **save_kwargs)

            base64_image = base64.b64encode(buffered.getvalue()).decode('ascii')

            live_preview = f"data:image/{format};base64,{base64_image}"

            live_preview = f"data:image/{opts.live_previews_image_format};base64,{base64_image}"

            id_live_preview = shared.state.id_live_preview

        else:

            live_preview = None

options_templates.update(options_section(('ui', "Live previews"), {

    "show_progressbar": OptionInfo(True, "Show progressbar"),

    "live_previews_enable": OptionInfo(True, "Show live previews of the created image"),

    "live_previews_format": OptionInfo("auto", "Live preview file format", gr.Radio, {"choices": ["auto", "jpeg", "png", "webp"]}),

    "live_previews_image_format": OptionInfo("png", "Live preview file format", gr.Radio, {"choices": ["jpeg", "png", "webp"]}),

    "show_progress_grid": OptionInfo(True, "Show previews of all images generated in a batch as a grid"),

    "show_progress_every_n_steps": OptionInfo(10, "Live preview display period", gr.Slider, {"minimum": -1, "maximum": 32, "step": 1}).info("in sampling steps - show new live preview image every N sampling steps; -1 = only show after completion of batch"),

    "show_progress_type": OptionInfo("Approx NN", "Live preview method", gr.Radio, {"choices": ["Full", "Approx NN", "Approx cheap"]}).info("Full = slow but pretty; Approx NN = fast but low quality; Approx cheap = super fast but terrible otherwise"),