Add screenshot support for eframe web (#5438)

This implements web support for taking screenshots in an eframe app (and
adds a nice demo).
It also updates the native screenshot implementation to work with the
wgpu gl backend.

The wgpu implementation is quite different than the native one because
we can't block to wait for the screenshot result, so instead I use a
channel to pass the result to a future frame asynchronously.

* Closes <https://github.com/emilk/egui/issues/5425>
* [x] I have followed the instructions in the PR template


https://github.com/user-attachments/assets/67cad40b-0384-431d-96a3-075cc3cb98fb

crates/egui-wgpu/src/texture_copy.wgsl

@@ -0,0 +1,43 @@
+struct VertexOutput {
+    @builtin(position) position: vec4<f32>,
+};
+
+var<private> positions: array<vec2f, 3> = array<vec2f, 3>(
+    vec2f(-1.0, -3.0),
+    vec2f(-1.0, 1.0),
+    vec2f(3.0, 1.0)
+);
+
+// meant to be called with 3 vertex indices: 0, 1, 2
+// draws one large triangle over the clip space like this:
+// (the asterisks represent the clip space bounds)
+//-1,1           1,1
+// ---------------------------------
+// |              *              .
+// |              *           .
+// |              *        .
+// |              *      .
+// |              *    .
+// |              * .
+// |***************
+// |            . 1,-1
+// |          .
+// |       .
+// |     .
+// |   .
+// |.
+@vertex
+fn vs_main(@builtin(vertex_index) vertex_index: u32) -> VertexOutput {
+    var result: VertexOutput;
+    result.position = vec4f(positions[vertex_index], 0.0, 1.0);
+    return result;
+}
+
+@group(0)
+@binding(0)
+var r_color: texture_2d<f32>;
+
+@fragment
+fn fs_main(vertex: VertexOutput) -> @location(0) vec4<f32> {
+    return textureLoad(r_color, vec2i(vertex.position.xy), 0);
+}