`CGRect::standardize` and `CGRect::is_empty` behavior for negative sizes

[andyleiserson]

The Apple documentation is somewhat ambiguous about how these should behave. https://stackoverflow.com/questions/35608638/accessing-cgrect-values-directly-vs-normalizing-them-in-swift-objective-c-rule and at least one GitHub code result suggest that standardizing a rectangle should move the origin if the size is negated (i.e. the rectangle should remain the same, vs. flip over the origin).

The current Rust implementation of CGRect::standardize only normalizes the size, leaving the origin unchanged.

Also, CGRect::is_empty returns !(self.size.width > 0.0 && self.size.height > 0.0), so will treat a negative-sized rectangle as empty.

[madsmtm]

This was introduced in #400.

I haven't given these much thought, but I agree that the following properties should hold for all values of rect:

CGRect::standardize(rect) == CGRectStandardize(rect)
CGRect::is_empty(rect) == CGRectIsEmpty(rect)

In theory, we should've been able to just make CGRectStandardize and CGRectIsEmpty be methods on CGRect, but that won't work because the CGRect type is defined in objc2-core-foundation, whereas the helper functions are only available in objc2-core-graphics (and require linking CoreGraphics too).