Disable default features from imageproc dependency

[JasperDeSutter]

...since it's pulling in all image formats (through image/default), nullifying the image_formats feature.
This makes rayon optional too, leaving more control to library users.

[hschimke]

How big a performance improvement is this in general terms? I have no objection to including it, there are probably a few places in the code where rayon might be useful as well. I'm going to have to evaluate though.

[JasperDeSutter]

I'm not sure it's worth it, I just noticed that turning off default features of imageproc also disabled rayon (it's on by default) so I didn't want to change that behavior in this PR.
Technically a feature isn't really needed for this because library users can depend on imageproc with the rayon feature enabled and that would enable it for usage inside rxing as well.

[hschimke]

I want to consider this one a little longer, I'm going to flag it for v0.9.3

[hschimke]

Hey @JasperDeSutter sorry it took me so long to get to this. Would you mind rebasing this on the current head? I'm good to merge after that.