Add ImageLoader::has_pending and wait_for_pending_images (#7030)

With kittest it was difficult to wait for images to be loaded before
taking a snapshot test.
This PR adds `Harness::with_wait_for_pending_images` (true by default)
which will cause `Harness::run` to sleep until all images are loaded (or
`HarnessBuilder::with_max_steps` is exceeded).

It also adds a new ImageLoader::has_pending and
BytesLoader::has_pending, which should be implemented if things are
loaded / decoded asynchronously.

It reverts https://github.com/emilk/egui/pull/6901 which was my previous
attempt to fix this (but this didn't work since only the tested crate is
compiled with cfg(test) and not it's dependencies)

crates/egui_kittest/tests/snapshots/should_wait_for_images.png

@@ -0,0 +1,3 @@
+version https://git-lfs.github.com/spec/v1
+oid sha256:6298e67d099002808d51e7494e4174adee66d7ef2880728126c1d761d1432372
+size 2145

crates/egui_kittest/tests/tests.rs

@@ -1,4 +1,4 @@
-use egui::Modifiers;
+use egui::{include_image, Modifiers, Vec2};
 use egui_kittest::Harness;
 use kittest::{Key, Queryable as _};
 
@@ -57,3 +57,29 @@ fn test_modifiers() {
     assert!(state.cmd_z_pressed, "Cmd+Z wasn't pressed");
     assert!(state.cmd_y_pressed, "Cmd+Y wasn't pressed");
 }
+
+#[test]
+fn should_wait_for_images() {
+    let mut harness = Harness::builder()
+        .with_size(Vec2::new(60.0, 120.0))
+        .build_ui(|ui| {
+            egui_extras::install_image_loaders(ui.ctx());
+            let size = Vec2::splat(30.0);
+            ui.label("Url:");
+            ui.add_sized(
+                size,
+                egui::Image::new(
+                    "https://raw.githubusercontent.com\
+                    /emilk/egui/refs/heads/main/crates/eframe/data/icon.png",
+                ),
+            );
+
+            ui.label("Include:");
+            ui.add_sized(
+                size,
+                egui::Image::new(include_image!("../../eframe/data/icon.png")),
+            );
+        });
+
+    harness.snapshot("should_wait_for_images");
+}