The API previously clobbered PIL.Image.MAX_IMAGE_PIXELS unconditionally on every call, so host applications (e.g. Paperless-NGX) that configured the PIL limit before invoking ocrmypdf.ocr() saw their setting silently overwritten with the 250 MP default. Make max_image_mpixels default to None and only apply the override when the caller explicitly sets it. The CLI default of 250 MP is unchanged. Fixes #1665