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.

Cargo.lock

@@ -1364,6 +1364,7 @@ dependencies = [
  "document-features",
  "egui",
  "egui-wgpu",
+ "egui_kittest",
  "image",
  "kittest",
  "pollster",