Function transform_environment 

pub fn transform_environment<K: EnvKey>(
    view: &ViewHandle,
    action: impl Fn(K::Value) -> K::Value + 'static,
) -> ViewHandle

.transformEnvironment(_:transform:) for any property this crate has a key for.

transform_environment_image_scale does this for one property, with the key path recovered from a modifier SwiftUI built. Every EnvKey already carries its own key path for @Environment to read through, so the same modifier works for all of them — the only thing that was missing was a way to write the value back, which is what crate::state::EnvKey::encode is.

// One size larger than whatever is in scope.
views::transform_environment::<ControlSizeKey>(&views::text("x"), |s| match s {
    ControlSize::Mini => ControlSize::Small,
    other => other,
});