Improve color picker cache (#886)

* colorpicker: try to maintain hue even when saturation goes to zero
* More consistent arguments to color types
* implement `Hash` for `Rgba`.

egui_demo_lib/src/apps/color_test.rs

@@ -156,7 +156,7 @@ impl ColorTest {
             ui,
             tex_allocator,
             RED,
-            (TRANSPARENT, Color32::from_rgba_premultiplied(0, 0, 255, 0)),
+            (TRANSPARENT, Color32::from_rgb_additive(0, 0, 255)),
         );
 
         ui.separator();