Add Harness::new_ui, Harness::fit_contents (#5301)

This adds a `Harness::new_ui`, which accepts a Ui closure and shows the
ui in a central panel. One big benefit is that this allows us to add a
fit_contents method that can run the ui closure with a sizing pass and
resize the "screen" based on the content size.

I also used this to add a snapshot test for the rendering_test at
different scales.

crates/egui_kittest/src/builder.rs

@@ -1,17 +1,18 @@
+use crate::app_kind::AppKind;
 use crate::Harness;
 use egui::{Pos2, Rect, Vec2};
 
 /// Builder for [`Harness`].
 pub struct HarnessBuilder {
     pub(crate) screen_rect: Rect,
-    pub(crate) dpi: f32,
+    pub(crate) pixels_per_point: f32,
 }
 
 impl Default for HarnessBuilder {
     fn default() -> Self {
         Self {
             screen_rect: Rect::from_min_size(Pos2::ZERO, Vec2::new(800.0, 600.0)),
-            dpi: 1.0,
+            pixels_per_point: 1.0,
         }
     }
 }
@@ -26,16 +27,18 @@ impl HarnessBuilder {
         self
     }
 
-    /// Set the DPI of the window.
+    /// Set the `pixels_per_point` of the window.
     #[inline]
-    pub fn with_dpi(mut self, dpi: f32) -> Self {
-        self.dpi = dpi;
+    pub fn with_pixels_per_point(mut self, pixels_per_point: f32) -> Self {
+        self.pixels_per_point = pixels_per_point;
         self
     }
 
     /// Create a new Harness with the given app closure.
     ///
-    /// The ui closure will immediately be called once to create the initial ui.
+    /// The app closure will immediately be called once to create the initial ui.
+    ///
+    /// If you don't need to create Windows / Panels, you can use [`HarnessBuilder::build_ui`] instead.
     ///
     /// # Example
     /// ```rust
@@ -50,6 +53,25 @@ impl HarnessBuilder {
     ///     });
     /// ```
     pub fn build<'a>(self, app: impl FnMut(&egui::Context) + 'a) -> Harness<'a> {
-        Harness::from_builder(&self, app)
+        Harness::from_builder(&self, AppKind::Context(Box::new(app)))
+    }
+
+    /// Create a new Harness with the given ui closure.
+    ///
+    /// The ui closure will immediately be called once to create the initial ui.
+    ///
+    /// If you need to create Windows / Panels, you can use [`HarnessBuilder::build`] instead.
+    ///
+    /// # Example
+    /// ```rust
+    /// # use egui_kittest::Harness;
+    /// let mut harness = Harness::builder()
+    ///     .with_size(egui::Vec2::new(300.0, 200.0))
+    ///     .build_ui(|ui| {
+    ///         ui.label("Hello, world!");
+    ///     });
+    /// ```
+    pub fn build_ui<'a>(self, app: impl FnMut(&mut egui::Ui) + 'a) -> Harness<'a> {
+        Harness::from_builder(&self, AppKind::Ui(Box::new(app)))
     }
 }