Add cornerCurve option to SurfaceAppearance

[zntfdr]

This PR adds support to customize the cornerCurve of the containerView

Screen.Recording.2020-12-19.at.11.23.44.AM.mov

I might be wrong, but it seems like the mask is not following the SurfaceAppearance's cornerCurve, initially this was because updateShadow() was called before updateCornerRadius(), but even after changing that the shadow seems to not adjust to the new cornerCurve. Any lead would be very welcome!

Also, I'm more than happy to close this PR if this can already be achieved in other ways, thank you :)

[scenee]

I agree with adding the new options! This is a point I was unable to make a decision to do it.
As you mentioned, we need to change the shadow path for the circular corners because UIBezierPath(roundedRect:..) API always use the continuous corners. So I would like to push a commit to update the shadow path. Should I create a feature branch to merge your PR?

[zntfdr]

Thank you @scenee ❤️

So I would like to push a commit to update the shadow path. Should I create a feature branch to merge your PR?

Please feel free to use this branch: as a maintainer you can push commits directly here 😃

[scenee]

I've added a commit to fix the shadow and its mask for circular corners.
https://user-images.githubusercontent.com/899487/103136760-87e15700-4706-11eb-955e-2e7406877e10.mov

[zntfdr]

Thank you @scenee 🙏🏻

[scenee]

@zntfdr, thank you too! We made a nice improvement 👍