WGPU: Add support for using WGPU textures in Slint

This adds a `unstable-wgpu-24` feature that exposes WGPU types in the
GraphicsAPI enum, adds `require_wgpu_24()` to the backend selector,
and adds a conversion from `wgpu::Texture` to `slint::Image`.

The `require_wgpu_24()` function in the selector will be extended in the
future (before the next release) to permit specifying additional aspects
of the WGPU configuration.

Co-Authored-By: Olivier Goffart <olivier.goffart@slint.dev>

internal/backends/selector/api.rs

@@ -1,6 +1,13 @@
 // Copyright © SixtyFPS GmbH <info@slint.dev>
 // SPDX-License-Identifier: GPL-3.0-only OR LicenseRef-Slint-Royalty-free-2.0 OR LicenseRef-Slint-Software-3.0
 
+#![warn(missing_docs)]
+
+/*!
+This module contains types that are public and re-exported in the slint-rs as well as the slint-interpreter crate as public API,
+in particular the `BackendSelector` type.
+*/
+
 use alloc::boxed::Box;
 use alloc::{format, string::String};
 
@@ -89,6 +96,18 @@ impl BackendSelector {
         self
     }
 
+    /// Adds the requirement to the selector that the backend must render using [WGPU](http://wgpu.rs).
+    /// Use this when you integrate other WGPU-based renderers with a Slint UI.
+    ///
+    /// *Note*: This function is behind a feature flag and may be removed or changed in future minor releases,
+    ///         as new major WGPU releases become available.
+    #[cfg(feature = "unstable-wgpu-24")]
+    #[must_use]
+    pub fn require_wgpu_24(mut self) -> Self {
+        self.requested_graphics_api = Some(RequestedGraphicsAPI::WGPU24);
+        self
+    }
+
     /// Adds the requirement that the selected renderer must match the given name. This is
     /// equivalent to setting the `SLINT_BACKEND=name` environment variable and requires
     /// that the corresponding renderer feature is enabled. For example, to select the Skia renderer,