Add feature to disable image crate's jpeg-rayon for wasm compatibility

[stanleydesu]

Loading JPEGs with xobject::image_from in WASM code causes a panic (e.g. image-rs/image#1496), as there isn't multithreading in WASM now and xobject::image_from uses image, which uses rayon for JPEG loading.

Could there be an option in lopdf for disabling image from using rayon (disabling example: image-rs/image#879 (comment)), such as a feature flag?

[J-F-Liu]

Yes, PR is welcome

[Heinenen]

According to image-rs/image#703 (comment), this issue should no longer occur.
@stanleydesu, can you confirm that the issue is resolved?
(I can't reproduce the error, but maybe I'm doing something wrong)

[Heinenen]

PS: rayon-rs/rayon#1019 is probably the thing that fixed it.