fix shadow rendering for tiny-skia backend

[T-256]

Fixes #2712

After commits bisecting, I found the issued came out from #2382 (commit 1e802e7).
This PR adds bounds_with_shadow to Quad.

I verified it by running cargo r -p custom_quad.

TODO

• Container shadows are inconsistent between wgpu and tiny-skia with custom theme #2339 (comment)
• In custom_quad example, after moving shadow down, hovering on sliders shouldn't destroy shadow.
• Prevent shadows beyond scrollable area.

[B0ney]

I don't think this is an optimal fix. Since now the software renderer would redraw everything every update, leading to much higher CPU usage.

[T-256]

I don't think this is an optimal fix

Agree.
Then I think there should be problem with layers determining. because I still have shadows shown beyond scrollable area even after with this PR.

[B0ney]

Looking at the issues you linked, I think it might be that the damage regions aren't taking the shadow's blur radius + offset into consideration.

This means that as the damaged areas are redrawn, because the regions aren't including the shadows, they won't get cleared properly.

Consequently, the renderer will draw over them (additively) after every update. This repeated stacking might explain why the shadows get darker.

[dtzxporter]

Looking at this, it may make sense to abstract the bounds calculation to "layout bounds" and "render bounds" where one deals with collision/layout, and the other is the bounds that will be modified when rendering, for dirty tracking, etc.

[T-256]

Looking at this, it may make sense to abstract the bounds calculation to "layout bounds" and "render bounds" where one deals with collision/layout, and the other is the bounds that will be modified when rendering, for dirty tracking, etc.

This approach makes sense to me, but I'm unsure how to implement it, could be great to have at least a PR to demonstrate how it effects on performance.

[T-256]

To reproduce scrollable bug with buttons with shadow:

Click to expand

use iced::widget::{button, column, container, scrollable, text};
use iced::{
    color,
    widget::button::{Status, Style},
    window::Settings,
    Shadow,
};
use iced::{Length, Theme};

pub trait StyleUtils {
    fn with_shadow(self, shadow: impl Into<Shadow>) -> Self;
}

impl StyleUtils for Style {
    fn with_shadow(self, shadow: impl Into<Shadow>) -> Self {
        Self {
            shadow: shadow.into(),
            ..self
        }
    }
}

#[derive(Default, Debug, Clone)]
enum Message {
    #[default]
    Test,
}

fn main() -> iced::Result {
    iced::application("shadow", |_: &mut i32, _| {}, view)
        .window(Settings {
            size: (300.0, 250.0).into(),
            resizable: false,
            ..Default::default()
        })
        .run()
}

fn view(_state: &i32) -> iced::Element<Message> {
    let mut c = column![];
    for _ in 0..10 {
        let shadow_button = button(text("foo").size(20)).style(|theme: &Theme, status: Status| {
            button::primary(theme, status).with_shadow(Shadow {
                color: color!(0x000000),
                offset: [2.0, 5.0].into(),
                blur_radius: 5.0,
            })
        });
        c = c.push(shadow_button.on_press(Message::Test));
    }
    container(scrollable(c.spacing(20).padding(20)))
        .center(Length::Fill)
        .padding(50)
        .into()
}