Add MemoizeImpure node and cache image base64 in graph (#1595)

* Cache base64 representation of images when converting to graphic group

* Fix build

* Fix build again

* Actually fix it this time

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Co-authored-by: Keavon Chambers <keavon@keavon.com>

node-graph/gcore/src/raster.rs

@@ -645,6 +645,7 @@ mod test {
 			width: 5,
 			height: 5,
 			data: vec![Color::from_rgbf32_unchecked(1., 0., 0.); 25],
+			base64_string: None,
 		});
 		let image = image.then(ImageRefNode::new());
 		let window = WindowNode::new(radius, image);