Blend modes (#252)

* Add backend for selecting layer blend mode

* Change dropdown input to support callback on change

* Add debug messages

* Fix canvas update for blend-modes

* Finish up and polish blend modes implementations

* Add changes from code review

Co-authored-by: Keavon Chambers <keavon@keavon.com>
Co-authored-by: Dennis Kobert <dennis@kobert.dev>

core/editor/src/frontend/layer_panel.rs

@@ -1,5 +1,8 @@
 use crate::document::LayerData;
-use document_core::{layers::LayerDataTypes, LayerId};
+use document_core::{
+	layers::{BlendMode, LayerDataTypes},
+	LayerId,
+};
 use serde::{Deserialize, Serialize};
 use std::fmt;
 
@@ -7,6 +10,7 @@ use std::fmt;
 pub struct LayerPanelEntry {
 	pub name: String,
 	pub visible: bool,
+	pub blend_mode: BlendMode,
 	pub layer_type: LayerType,
 	pub layer_data: LayerData,
 	pub path: Vec<LayerId>,