Loading a jpg image causes a panic in WASM

[francesco-cattoglio]

Describe the bug
Loading a jpg file in wasm build causes a panic.

To Reproduce
Run the image.rs example, but instead of loading a png, try loading a jpg (such as the attached one) and run with cargo web start

Running the example in native build will work with no issues instead.

Environment and versions (please complete the following information):

• Environment: Ubuntu + Firefox, but it should not matter
• Rust compiler version: Rust stable 1.35
• Quicksilver version: latest

Notes:
I tracked down the origin of the problem, and it lies in the jpeg-decoder package: by default it uses rayon, but rayon is not fully supported by wasm yet: it does compile, but it panics on usage. See also image-rs/image#879 and image-rs/image#724

It should be really easy to fix, just by modifying the Cargo.toml. However when I tried playing around with my local quicksilver version, no matter what changes I did to Cargo.toml, they were ignored, so I was unable to create a fix... sorry, I am still a rust noob! 😛
OTOH, if I use my fork of jpeg-decoder and manually disable the code path using rayon, everything works, so I am 100% confident: usage of rayon in the image/jpeg-decoder package is the source of the panic.

[ryanisaacg]

That is indeed a problem! I guess the way to fix that is to disable Rayon in image on web targets. At some point this week I'll try to take a look and get a fix out for that.