Improve egui_kittest documentation and update CONTRIBUTING.md (#5296)

This should hopefully make it easier for egui contributors and
egui_kittest users to understand the snapshot tests


* [X] I have followed the instructions in the PR template

crates/egui_kittest/src/snapshot.rs

@@ -80,6 +80,9 @@ impl Display for SnapshotError {
 }
 
 /// Image snapshot test.
+/// The snapshot will be saved under `tests/snapshots/{name}.png`.
+/// The new image from the last test run will be saved under `tests/snapshots/{name}.new.png`.
+/// If new image didn't match the snapshot, a diff image will be saved under `tests/snapshots/{name}.diff.png`.
 ///
 /// # Errors
 /// Returns a [`SnapshotError`] if the image does not match the snapshot or if there was an error
@@ -170,6 +173,9 @@ fn maybe_update_snapshot(
 }
 
 /// Image snapshot test.
+/// The snapshot will be saved under `tests/snapshots/{name}.png`.
+/// The new image from the last test run will be saved under `tests/snapshots/{name}.new.png`.
+/// If new image didn't match the snapshot, a diff image will be saved under `tests/snapshots/{name}.diff.png`.
 ///
 /// # Panics
 /// Panics if the image does not match the snapshot or if there was an error reading or writing the
@@ -187,6 +193,9 @@ pub fn image_snapshot(current: &image::RgbaImage, name: &str) {
 #[cfg(feature = "wgpu")]
 impl Harness<'_> {
     /// Render a image using a default [`crate::wgpu::TestRenderer`] and compare it to the snapshot.
+    /// The snapshot will be saved under `tests/snapshots/{name}.png`.
+    /// The new image from the last test run will be saved under `tests/snapshots/{name}.new.png`.
+    /// If new image didn't match the snapshot, a diff image will be saved under `tests/snapshots/{name}.diff.png`.
     ///
     /// # Errors
     /// Returns a [`SnapshotError`] if the image does not match the snapshot or if there was an error
@@ -198,6 +207,9 @@ impl Harness<'_> {
     }
 
     /// Render a image using a default [`crate::wgpu::TestRenderer`] and compare it to the snapshot.
+    /// The snapshot will be saved under `tests/snapshots/{name}.png`.
+    /// The new image from the last test run will be saved under `tests/snapshots/{name}.new.png`.
+    /// If new image didn't match the snapshot, a diff image will be saved under `tests/snapshots/{name}.diff.png`.
     ///
     /// # Panics
     /// Panics if the image does not match the snapshot or if there was an error reading or writing the

crates/egui_kittest/src/wgpu.rs

@@ -6,6 +6,7 @@ use image::RgbaImage;
 use std::iter::once;
 use wgpu::Maintain;
 
+/// Utility to render snapshots from a [`Harness`] using [`egui_wgpu`].
 pub struct TestRenderer {
     device: wgpu::Device,
     queue: wgpu::Queue,
@@ -19,6 +20,7 @@ impl Default for TestRenderer {
 }
 
 impl TestRenderer {
+    /// Create a new [`TestRenderer`] using a default [`wgpu::Instance`].
     pub fn new() -> Self {
         let instance = wgpu::Instance::new(InstanceDescriptor::default());
 
@@ -39,6 +41,7 @@ impl TestRenderer {
         Self::create(device, queue)
     }
 
+    /// Create a new [`TestRenderer`] using the provided [`wgpu::Device`] and [`wgpu::Queue`].
     pub fn create(device: wgpu::Device, queue: wgpu::Queue) -> Self {
         Self {
             device,
@@ -47,13 +50,20 @@ impl TestRenderer {
         }
     }
 
+    /// Enable or disable dithering.
+    ///
+    /// Disabled by default.
     #[inline]
     pub fn with_dithering(mut self, dithering: bool) -> Self {
         self.dithering = dithering;
         self
     }
 
+    /// Render the [`Harness`] and return the resulting image.
     pub fn render(&mut self, harness: &Harness<'_>) -> RgbaImage {
+        // We need to create a new renderer each time we render, since the renderer stores
+        // textures related to the Harnesses' egui Context.
+        // Calling the renderer from different Harnesses would cause problems if we store the renderer.
         let mut renderer = egui_wgpu::Renderer::new(
             &self.device,
             TextureFormat::Rgba8Unorm,