Switch the Color struct back to storing unassociated alpha (#4518)

* Switch Color struct back to storing unassociated alpha

* Address review feedback

* Update the Invert node and legacy image migration for straight alpha and add round-trip tests

---------

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

editor/src/messages/portfolio/document/document_message_handler.rs

@@ -802,8 +802,6 @@ impl MessageHandler<DocumentMessage, DocumentMessageContext<'_>> for DocumentMes
 				parent_and_insert_index,
 				place_at_origin,
 			} => {
-				// All the image's pixels have been converted to 0..=1, linear, and premultiplied by `Color::from_rgba8_srgb`
-
 				let layer_parent = self.new_layer_parent(true);
 				let image_size = DVec2::new(image.width as f64, image.height as f64);
 

editor/src/messages/portfolio/document_migration.rs

@@ -2123,7 +2123,9 @@ fn migrate_node(node_id: &NodeId, node: &DocumentNode, network_path: &[NodeId],
 			_ => None,
 		});
 
-		if let Some(image) = image {
+		if let Some(mut image) = image {
+			// Legacy embedded pixel data is premultiplied, so restore straight alpha before encoding it
+			image.data.iter_mut().for_each(|pixel| *pixel = pixel.to_unassociated_alpha());
 			let hash = document.resources.embedded.store(Resource::new(image.to_png()));
 
 			let resource_id = ResourceId::new();