Add an opacity option for the Eraser's tool options

This lets the user change the strength of the tool. When it has an opacity of 255, the eraser will completely delete the underlying pixels. If it has less than 255, it will simply subtruct that value from the underlying pixel's alpha.

src/Tools/Eraser.gd

@@ -9,12 +9,12 @@ var _changed := false
 class EraseOp extends Drawer.ColorOp:
 	var changed := false
 
-
-	func process(_src: Color, _dst: Color) -> Color:
+	func process(_src: Color, dst: Color) -> Color:
 		changed = true
-#		dst.a -= src.a * strength
-#		return dst
-		return Color(0, 0, 0, 0)
+		dst.a -= strength
+		if dst.a <= 0:
+			dst = Color(0, 0, 0, 0)
+		return dst
 
 
 func _init() -> void:
@@ -23,6 +23,17 @@ func _init() -> void:
 	_clear_image.fill(Color(0, 0, 0, 0))
 
 
+func get_config() -> Dictionary:
+	var config := .get_config()
+	config["strength"] = _strength
+	return config
+
+
+func set_config(config : Dictionary) -> void:
+	.set_config(config)
+	_strength = config.get("strength", _strength)
+
+
 func draw_start(position : Vector2) -> void:
 	Global.canvas.selection.transform_content_confirm()
 	update_mask()
@@ -77,3 +88,15 @@ func _draw_brush_image(image : Image, src_rect: Rect2, dst: Vector2) -> void:
 	var images := _get_selected_draw_images()
 	for draw_image in images:
 		draw_image.blit_rect_mask(_clear_image, image, src_rect, dst)
+
+
+func _on_Opacity_value_changed(value: float) -> void:
+	_strength = value / 255
+	update_config()
+	save_config()
+
+
+func update_config() -> void:
+	.update_config()
+	$OpacitySpinBox.value = _strength * 255
+	$OpacitySlider.value = _strength * 255

src/Tools/Eraser.tscn

@@ -5,3 +5,50 @@
 
 [node name="ToolOptions" instance=ExtResource( 1 )]
 script = ExtResource( 2 )
+
+[node name="OpacityLabel" type="Label" parent="." index="3"]
+margin_top = 74.0
+margin_right = 116.0
+margin_bottom = 88.0
+text = "Opacity:"
+align = 1
+
+[node name="OpacitySpinBox" type="SpinBox" parent="." index="4"]
+margin_left = 21.0
+margin_top = 92.0
+margin_right = 95.0
+margin_bottom = 116.0
+mouse_default_cursor_shape = 2
+size_flags_horizontal = 4
+max_value = 255.0
+value = 255.0
+align = 1
+__meta__ = {
+"_edit_use_anchors_": false
+}
+
+[node name="OpacitySlider" type="HSlider" parent="." index="5"]
+margin_left = 12.0
+margin_top = 120.0
+margin_right = 104.0
+margin_bottom = 136.0
+rect_min_size = Vector2( 92, 0 )
+mouse_default_cursor_shape = 2
+size_flags_horizontal = 4
+max_value = 255.0
+value = 255.0
+
+[node name="PixelPerfect" parent="." index="6"]
+margin_top = 140.0
+margin_bottom = 164.0
+
+[node name="EmptySpacer" parent="." index="8"]
+margin_top = 168.0
+margin_bottom = 180.0
+
+[node name="Mirror" parent="." index="9"]
+margin_top = 184.0
+margin_bottom = 201.0
+
+[connection signal="value_changed" from="OpacitySpinBox" to="." method="_on_Opacity_value_changed"]
+[connection signal="value_changed" from="OpacitySlider" to="." method="_on_Opacity_value_changed"]