Make properties of CGPoint, CGSize and CGRect vars instead of lets

[mortenbekditlevsen]

And initialized array used by CGAffineTransform concatenation

[mortenbekditlevsen]

Note:
The rotated(by:) implementation appears to be a copy of the init(rotationAngle:) - it doesn't take the current matrix values into account at all.
Would it suffice to return self.concatenating(Self(rotationAngle: angle)) ?

[MaxDesiatov]

Long term we should use Foundation types directly instead of our own stubs, but for now this seems legit.

[carson-katri]

We should probably separate it out to it's own product and do

#if canImport(CoreGraphics)
@_exported import CoreGraphics
#else
...
#endif

since I doubt CG will be available on the web, for instance.

[MaxDesiatov]

CoreGraphics itself is not available, but all of the types and some of their extensions in CGStubs are already available in Foundation for WASI. I've tried using that instead of our own reimplementation, but that broke some of our Shape SVG rendering. We need to add unit tests to thoroughly cover that area and make the switch. We already link with Foundation for Bundle required by the Image type anyway.