Container doesnot request_paint when env value is changed

[maan2003]

example:

// Copyright 2021 The Druid Authors.
//
// Licensed under the Apache License, Version 2.0 (the "License");
// you may not use this file except in compliance with the License.
// You may obtain a copy of the License at
//
//     http://www.apache.org/licenses/LICENSE-2.0
//
// Unless required by applicable law or agreed to in writing, software
// distributed under the License is distributed on an "AS IS" BASIS,
// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
// See the License for the specific language governing permissions and
// limitations under the License.

use druid::widget::prelude::*;
use druid::widget::{Flex, SizedBox, Switch};
use druid::{AppLauncher, Color, Key, WidgetExt, WindowDesc};

pub fn main() {
    let main_window = WindowDesc::new(build_root_widget());

    AppLauncher::with_window(main_window)
        .log_to_console()
        .launch(true)
        .expect("Failed to launch application");
}

fn build_root_widget() -> impl Widget<bool> {
    Flex::column()
        .with_child(Switch::new())
        .with_default_spacer()
        .with_child(changing_bg())
}

fn changing_bg() -> druid::widget::EnvScope<bool, druid::widget::Container<bool>> {
    const BG: Key<Color> = Key::new("druid-test.bg-color");
    SizedBox::empty()
        .width(500.)
        .height(500.)
        .background(BG)
        .env_scope(|env, data| env.set(BG, if *data { Color::RED } else { Color::BLUE }))
}