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/src/builder.rs

@@ -11,6 +11,7 @@ pub struct HarnessBuilder<State = ()> {
     pub(crate) step_dt: f32,
     pub(crate) state: PhantomData<State>,
     pub(crate) renderer: Box<dyn TestRenderer>,
+    pub(crate) wait_for_pending_images: bool,
 }
 
 impl<State> Default for HarnessBuilder<State> {
@@ -22,6 +23,7 @@ impl<State> Default for HarnessBuilder<State> {
             renderer: Box::new(LazyRenderer::default()),
             max_steps: 4,
             step_dt: 1.0 / 4.0,
+            wait_for_pending_images: true,
         }
     }
 }
@@ -63,6 +65,19 @@ impl<State> HarnessBuilder<State> {
         self
     }
 
+    /// Should we wait for pending images?
+    ///
+    /// If `true`, [`Harness::run`] and related methods will check if there are pending images
+    /// (via [`egui::Context::has_pending_images`]) and sleep for [`Self::with_step_dt`] up to
+    /// [`Self::with_max_steps`] times.
+    ///
+    /// Default: `true`
+    #[inline]
+    pub fn with_wait_for_pending_images(mut self, wait_for_pending_images: bool) -> Self {
+        self.wait_for_pending_images = wait_for_pending_images;
+        self
+    }
+
     /// Set the [`TestRenderer`] to use for rendering.
     ///
     /// By default, a [`LazyRenderer`] is used.