Add kittest.toml config file (#7643)

* part of https://github.com/rerun-io/rerun/issues/10991

---------

Co-authored-by: lucasmerlin <8009393+lucasmerlin@users.noreply.github.com>

crates/egui_kittest/src/snapshot.rs

@@ -1,28 +1,35 @@
-use crate::Harness;
-use image::ImageError;
 use std::fmt::Display;
 use std::io::ErrorKind;
 use std::path::PathBuf;
 
+use image::ImageError;
+
+use crate::{Harness, config::config};
+
 pub type SnapshotResult = Result<(), SnapshotError>;
 
 #[non_exhaustive]
 #[derive(Clone, Debug)]
 pub struct SnapshotOptions {
     /// The threshold for the image comparison.
-    /// The default is `0.6` (which is enough for most egui tests to pass across different
-    /// wgpu backends).
+    ///
+    /// Can be configured via kittest.toml. The fallback is `0.6` (which is enough for most egui
+    /// tests to pass across different wgpu backends).
     pub threshold: f32,
 
     /// The number of pixels that can differ before the snapshot is considered a failure.
+    ///
     /// Preferably, you should use `threshold` to control the sensitivity of the image comparison.
     /// As a last resort, you can use this to allow a certain number of pixels to differ.
-    /// If `None`, the default is `0` (meaning no pixels can differ).
-    /// If `Some`, the value can be set per OS
+    /// Can be configured via kittest.toml. The fallback is `0` (meaning no pixels can differ).
     pub failed_pixel_count_threshold: usize,
 
     /// The path where the snapshots will be saved.
-    /// The default is `tests/snapshots`.
+    ///
+    /// This is relative to the current working directory (usually the crate root when
+    /// running tests).
+    ///
+    /// Can be configured via kittest.toml. The fallback is `tests/snapshots`.
     pub output_path: PathBuf,
 }
 
@@ -30,7 +37,9 @@ pub struct SnapshotOptions {
 ///
 /// This is useful if you want to set different thresholds for different operating systems.
 ///
-/// The default values are 0 / 0.0
+/// [`OsThreshold::default`] gets the default from the config file (`kittest.toml`).
+/// For `usize`, it's the `failed_pixel_count_threshold` value.
+/// For `f32`, it's the `threshold` value.
 ///
 /// Example usage:
 /// ```no_run
@@ -53,12 +62,36 @@ pub struct OsThreshold<T> {
     pub fallback: T,
 }
 
+impl Default for OsThreshold<usize> {
+    /// Returns the default `failed_pixel_count_threshold` as configured in `kittest.toml`
+    ///
+    /// The fallback is `0`.
+    fn default() -> Self {
+        config().os_failed_pixel_count_threshold()
+    }
+}
+
+impl Default for OsThreshold<f32> {
+    /// Returns the default `threshold` as configured in `kittest.toml`
+    ///
+    /// The fallback is `0.6`.
+    fn default() -> Self {
+        config().os_threshold()
+    }
+}
+
 impl From<usize> for OsThreshold<usize> {
     fn from(value: usize) -> Self {
         Self::new(value)
     }
 }
 
+impl From<f32> for OsThreshold<f32> {
+    fn from(value: f32) -> Self {
+        Self::new(value)
+    }
+}
+
 impl<T> OsThreshold<T>
 where
     T: Copy,
@@ -123,9 +156,9 @@ impl From<OsThreshold<Self>> for f32 {
 impl Default for SnapshotOptions {
     fn default() -> Self {
         Self {
-            threshold: 0.6,
-            output_path: PathBuf::from("tests/snapshots"),
-            failed_pixel_count_threshold: 0, // Default is 0, meaning no pixels can differ
+            threshold: config().threshold(),
+            output_path: config().output_path(),
+            failed_pixel_count_threshold: config().failed_pixel_count_threshold(),
         }
     }
 }