non-uniform border radi for quads

[rksm]

Hello, I would like to add support for a CSS-style border radius for quads and quad based-widgets, e.g the button. This means being able to specify different border radius values for the separate corners.

The implementation isn't complicated but this will have an effect on the widget API for button styles and pick lists which is why I'm opening this discussion. Instead of having a f32 for the border radius would either be [f32; 4] or a type wrapping that like pub struct BorderRadius([f32; 4]);. The latter would have the advantage of allowing to implement From<f32> which will make it a bit more ergonomic.

So two questions:

1. Would non-uniform border radi be a feature that would be accepted?
2. What would the preferred way to specify that be, in particular for button styles?

[rksm]

PR for this has been opened here #1506