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_demo_lib/tests/snapshots/rendering_test/dpi_1.00.png

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