Fix group layers blending twice in Canvas.update_texture() when their blend mode is set to passthrough

This solves a part of #1166

src/Autoload/DrawingAlgos.gd

@@ -56,7 +56,7 @@ func blend_layers(
 		if DisplayServer.get_name() == "headless":
 			blend_layers_headless(image, project, layer, cel, origin)
 		else:
-			if layer is GroupLayer and layer.blend_mode != BaseLayer.BlendModes.PASS_THROUGH:
+			if layer.is_blender():
 				var cel_image := (layer as GroupLayer).blend_children(frame)
 				textures.append(cel_image)
 			else:

src/Classes/Layers/BaseLayer.gd

@@ -355,3 +355,10 @@ func accepts_child(_layer: BaseLayer) -> bool:
 ## Returns an instance of the layer button that will be added to the timeline.
 func instantiate_layer_button() -> Node:
 	return Global.layer_button_node.instantiate()
+
+
+## Returns [code]true[/code] if the layer is responsible for blending other layers.
+## Currently only returns [code]true[/code] with [GroupLayer]s, when their
+## blend mode is set to something else rather than [enum BlendModes.PASS_THROUGH].
+func is_blender() -> bool:
+	return false

src/Classes/Layers/GroupLayer.gd

@@ -167,3 +167,7 @@ func set_name_to_default(number: int) -> void:
 
 func accepts_child(_layer: BaseLayer) -> bool:
 	return true
+
+
+func is_blender() -> bool:
+	return blend_mode != BlendModes.PASS_THROUGH

src/UI/Canvas/Canvas.gd

@@ -126,7 +126,7 @@ func update_texture(
 			return
 		var layer := project.layers[layer_i].get_blender_ancestor()
 		var cel_image: Image
-		if layer is GroupLayer:
+		if layer.is_blender():
 			cel_image = layer.blend_children(
 				project.frames[project.current_frame], Vector2i.ZERO, Global.display_layer_effects
 			)
@@ -217,7 +217,7 @@ func _update_texture_array_layer(
 	var ordered_index := project.ordered_layers[layer.index]
 	var cel := project.frames[project.current_frame].cels[layer.index]
 	var include := true
-	if layer is GroupLayer and layer.blend_mode != BaseLayer.BlendModes.PASS_THROUGH:
+	if layer.is_blender():
 		cel_image.copy_from(
 			layer.blend_children(
 				project.frames[project.current_frame],

src/UI/Canvas/CanvasPreview.gd

@@ -85,7 +85,7 @@ func _draw_layers() -> void:
 		var cel := current_cels[i]
 		var layer := project.layers[i]
 		var cel_image: Image
-		if layer is GroupLayer and layer.blend_mode != BaseLayer.BlendModes.PASS_THROUGH:
+		if layer.is_blender():
 			cel_image = layer.blend_children(
 				current_frame, Vector2i.ZERO, Global.display_layer_effects
 			)

src/UI/Dialogs/ImageEffects/OffsetImage.gd

@@ -102,7 +102,7 @@ func blend_layers(
 				include = false
 		var cel := frame.cels[ordered_index]
 		var cel_image: Image
-		if layer is GroupLayer and layer.blend_mode != BaseLayer.BlendModes.PASS_THROUGH:
+		if layer.is_blender():
 			cel_image = (layer as GroupLayer).blend_children(frame)
 		else:
 			cel_image = layer.display_effects(cel)