Evaluate dif for inequalities only if arguments are reals

fixes #8288
> "Explicit is better than implicit"

Although practicality beats purity.  In this case, having to sometimes make symbols real to get the old behavior in geometry would feel like a regression. I think the idea below is better.

> use Dummies with appropriate assumptions internally

Yes, if you want to keep the code handling pure. We could have a "recast_real" function like "posify" that does this for any vanilla variables.