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
1c89cacfef