Add support for gradients with midpoints and add draggable diamonds to the color picker dialog (#3813)

* Refactor GradientStops to use struct-of-arrays and include midpoint

* Implement interaction and rendering

* Make color picker saturation-value color picking snap to original position and show both axis lines

Make color picker saturation-value color picking snap to original position and show both axis lines

* Add graphite:midpoint attribute to SVG exports

* Add graphite:midpoint parsing to SVG importer

node-graph/nodes/raster/src/adjust.rs

@@ -41,7 +41,7 @@ mod adjust_std {
 	}
 	impl Adjust<Color> for GradientStops {
 		fn adjust(&mut self, map_fn: impl Fn(&Color) -> Color) {
-			for (_, color) in self.iter_mut() {
+			for color in self.color.iter_mut() {
 				*color = map_fn(color);
 			}
 		}