Can't re-export features of optional dependency

[Diggsey]

As far as I can tell, there's no way to re-export features of an optional dependency.

Take these crates:
rayon - only compiles on targets with threads
jpeg_decoder - has a default feature "rayon" to enable faster decoding where possible
image - has a default feature "jpeg" which enables the jpeg decoder

Now the first problem is that image has an unconditional dependency on rayon. This isn't great, but it should be possible to fix by re-exporting a "rayon" feature from image.

However this doesn't work, because the "rayon" feature would have to automatically include the "jpeg_decoder" dependency, even if the "jpeg" feature is not enabled.

There should be a way to enable a dependency or dependency feature based on an intersection of top-level features.

[alexcrichton]

I believe this is a dupe of #3494, so closing in favor of that