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/wgpu.rs

@@ -128,7 +128,7 @@ impl TestRenderer {
                         view: &texture_view,
                         resolve_target: None,
                         ops: wgpu::Operations {
-                            load: wgpu::LoadOp::Clear(wgpu::Color::BLACK),
+                            load: wgpu::LoadOp::Clear(wgpu::Color::TRANSPARENT),
                             store: StoreOp::Store,
                         },
                     })],