This was easy to implement. I like the result, I think `borderRadius.zero` -> `borderRadius.circular` makes a nice transition, and many places (like macOS) use an effect similar to this PR, while Google doesn't use anywhere (yet). I'm curious if it is going to break goldens or google testing.

<img width="954" alt="ttt" src="https://user-images.githubusercontent.com/351125/229918871-9f2ab851-6b41-44f5-80b9-c7928a8c0014.png">

What do you think? cc @HansMuller @gspencergoog. Is this something you want, should I ask the community, or do you prefer the current one?

Side effects:
- This makes strokeAlign work with `UnderlineInputBorder` (TODO: fix `drawLine` when borderRadius is zero).
- This is faster than the current implementation (clip is slow on Skia). 🚀 
- We could just call `BoxBorder._paintNonUniformBorder` (if it weren't private). Single LOC implementation.
- Web does this by default:
![image](https://user-images.githubusercontent.com/351125/233448671-90ce62ff-be91-40ca-8007-e82b57f3272e.png)
- Apparently no tests fail and most usages around (via code search) seem to be without a borderRadius.