Add snapping to endpoints and stops in the Gradient tool (#3732)

* snapping

* Cleanup

* fix

* Fix snapping failing sometimes on newly drawn gradient lines

* Code cleanup

---------

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

editor/src/messages/tool/common_functionality/snapping.rs

@@ -251,15 +251,23 @@ impl SnapManager {
 	pub fn update_indicator(&mut self, snapped_point: SnappedPoint) {
 		self.indicator = snapped_point.is_snapped().then_some(snapped_point);
 	}
+
 	pub fn clear_indicator(&mut self) {
 		self.indicator = None;
 	}
+
 	pub fn preview_draw(&mut self, snap_data: &SnapData, mouse: DVec2) {
 		let point = SnapCandidatePoint::handle(snap_data.document.metadata().document_to_viewport.inverse().transform_point2(mouse));
 		let snapped = self.free_snap(snap_data, &point, SnapTypeConfiguration::default());
 		self.update_indicator(snapped);
 	}
 
+	pub fn preview_draw_gradient(&mut self, snap_data: &SnapData, mouse: DVec2) {
+		let point = SnapCandidatePoint::gradient_handle(snap_data.document.metadata().document_to_viewport.inverse().transform_point2(mouse));
+		let snapped = self.free_snap(snap_data, &point, SnapTypeConfiguration::default());
+		self.update_indicator(snapped);
+	}
+
 	pub fn indicator_pos(&self) -> Option<DVec2> {
 		self.indicator.as_ref().map(|point| point.snapped_point_document)
 	}

editor/src/messages/tool/common_functionality/snapping/layer_snapper.rs

@@ -453,6 +453,10 @@ impl SnapCandidatePoint {
 		Self::new_source(document_point, SnapSource::Path(PathSnapSource::AnchorPointWithFreeHandles))
 	}
 
+	pub fn gradient_handle(document_point: DVec2) -> Self {
+		Self::new_source(document_point, SnapSource::Gradient(GradientSnapSource::Endpoint))
+	}
+
 	pub fn handle_neighbors(document_point: DVec2, neighbors: impl Into<Vec<DVec2>>) -> Self {
 		let mut point = Self::new_source(document_point, SnapSource::Path(PathSnapSource::AnchorPointWithFreeHandles));
 		point.neighbors = neighbors.into();