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/document/src/operation.rs

@@ -1,6 +1,6 @@
 use crate::{
 	color::Color,
-	layers::{style, Layer},
+	layers::{style, BlendMode, Layer},
 	LayerId,
 };
 
@@ -72,6 +72,10 @@ pub enum Operation {
 	ToggleVisibility {
 		path: Vec<LayerId>,
 	},
+	SetLayerBlendMode {
+		path: Vec<LayerId>,
+		blend_mode: BlendMode,
+	},
 	FillLayer {
 		path: Vec<LayerId>,
 		color: Color,