Add command line option to choose a starting scene in the `testbed_*' examples

[panpanpro888]

Objective

• Fixes Add commandline options to the testbed_* examples to choose the starting scene #22218

Solution

• Use argh to check if you passed a scene name, and if you did, start on that one instead of the default

Testing

• Tested on my local machine, I don't see a reason why it would change between different machines and/or platforms

[apekros]

We can simplify the match statements

[apekros]

Suggested change

	match args.scene {
	None => app.init_state::<Scene>(),
	Some(Scene::Image) => app.insert_state(Scene::Image),
	Some(Scene::Text) => app.insert_state(Scene::Text),
	Some(Scene::Grid) => app.insert_state(Scene::Grid),
	Some(Scene::Borders) => app.insert_state(Scene::Borders),
	Some(Scene::BoxShadow) => app.insert_state(Scene::BoxShadow),
	Some(Scene::TextWrap) => app.insert_state(Scene::TextWrap),
	Some(Scene::Overflow) => app.insert_state(Scene::Overflow),
	Some(Scene::Slice) => app.insert_state(Scene::Slice),
	Some(Scene::LayoutRounding) => app.insert_state(Scene::LayoutRounding),
	Some(Scene::LinearGradient) => app.insert_state(Scene::LinearGradient),
	Some(Scene::RadialGradient) => app.insert_state(Scene::RadialGradient),
	Some(Scene::Transformations) => app.insert_state(Scene::Transformations),
	#[cfg(feature = "bevy_ui_debug")]
	Some(Scene::DebugOutlines) => app.insert_state(Scene::DebugOutlines),
	Some(Scene::ViewportCoords) => app.insert_state(Scene::ViewportCoords),
	};
	match args.scene {
	None => app.init_state::<Scene>(),
	Some(scene) => app.insert_state(scene),
	};

[apekros]

Suggested change

	match args.scene {
	None => app.init_state::<Scene>(),
	Some(Scene::Light) => app.insert_state(Scene::Light),
	Some(Scene::Bloom) => app.insert_state(Scene::Bloom),
	Some(Scene::Gltf) => app.insert_state(Scene::Gltf),
	Some(Scene::Animation) => app.insert_state(Scene::Animation),
	Some(Scene::Gizmos) => app.insert_state(Scene::Gizmos),
	Some(Scene::GltfCoordinateConversion) => app.insert_state(Scene::GltfCoordinateConversion),
	};
	match args.scene {
	None => app.init_state::<Scene>(),
	Some(scene) => app.insert_state(scene),
	};

[apekros]

Suggested change

	match args.scene {
	None => app.init_state::<Scene>(),
	Some(Scene::Shapes) => app.insert_state(Scene::Shapes),
	Some(Scene::Bloom) => app.insert_state(Scene::Bloom),
	Some(Scene::Text) => app.insert_state(Scene::Text),
	Some(Scene::Sprite) => app.insert_state(Scene::Sprite),
	Some(Scene::SpriteSlicing) => app.insert_state(Scene::SpriteSlicing),
	Some(Scene::Gizmos) => app.insert_state(Scene::Gizmos),
	};
	match args.scene {
	None => app.init_state::<Scene>(),
	Some(scene) => app.insert_state(scene),
	};