Add a checkered background to transparent artboards and the infinite canvas (#4022)

* Add checkered transparency rendering to infinite canvas and artboards

* Enable artboard clipping by default

* Make new infinite canvas documents begin with a white background layer

* Remove the export dialog's transparency option now that it's redundant

* Make exporting transparent JPGs use white not black

* Code review

node-graph/nodes/graphic/src/artboard.rs

@@ -31,6 +31,7 @@ pub async fn create_artboard<T: IntoGraphicTable + 'n>(
 	/// Color of the artboard background. Only positive integers are valid.
 	background: Table<Color>,
 	/// Whether to cut off the contained content that extends outside the artboard, or keep it visible.
+	#[default(true)]
 	clip: bool,
 ) -> Table<Artboard> {
 	let location = location.as_ivec2();