Make the "objdetect" feature enable the "dnn" and "features" features.

[de-vri-es]

As an alternative to #724: since building the opencv crate with "objdetect" feature fails to build with the "dnn" and "features" features. The "features" module is a hard requirement, and the "dnn" module is an optional dependency. However, if the "dnn" module is enabled in the opencv build, it becomes a hard requirement too because of the way the binding generator works.

And since unavailable modules are silently ignored, it seems better to add the dependency than to omit it.

[twistedfall]

Yeah, this approach would be my first choice. But can you confirm that it solves the problems you were encountering?

[de-vri-es]

It does fix it :)

[twistedfall]

Great to hear. Then let's leave it as is, the changes in the previous PR did feel too intrusive, they would introduce another target that would need to be followed as OpenCV evolves and I try to avoid that. I'll try to prepare the new (minor) release this week.

[de-vri-es]

Yeah, I agree. I was under the impression that enabling the dnn feature would make the dnn module required. But since it doesn't, this is much simpler, and less error prone :)