[Fee0]

iced_wgpu picks the surface alpha mode once at startup and reuses it in every configure_surface. But the supported modes can change during a surface's lifetime: on Windows with Vulkan and vsync off, an NVIDIA driver (RTX 3060 Ti, 595.71) offers [Opaque, PreMultiplied] for the first two swapchains and only [Opaque] afterwards. The next resize then panics:

In Surface::configure
  Requested alpha mode PreMultiplied is not in the list of supported alpha modes: [Opaque]

This PR queries the surface capabilities on every configure and picks the mode from what is supported right now (same preference as before: PreMultiplied, else Auto). This also fixes multi-window apps reusing the mode probed on the first window.

Repro (Windows, Vulkan)

use iced::backend::Api;
use iced::{Backend, Element, Settings, Size, Task, Widget as _, time, widget::text, window};
use std::time::Duration;

fn main() -> iced::Result {
    let settings = Settings {
        backend: Backend::Hardware(Api::Vulkan),
        vsync: false,
        ..Settings::default()
    };

    iced::application(|| 0, update, view)
        .settings(settings)
        .subscription(|_| time::every(Duration::from_millis(100)).map(|_| ()))
        .run()
}

fn update(resizes: &mut u32, _tick: ()) -> Task<()> {
    *resizes += 1;
    let size = if *resizes % 2 == 0 { Size::new(800.0, 600.0) } else { Size::new(600.0, 400.0) };
    window::latest().and_then(move |id| window::resize(id, size))
}

fn view(resizes: &u32) -> Element<'_, ()> {
    text!("{resizes} resizes").boxed()
}