Add support for InputEventMouseMotion.pen_inverted (#966)

* Add support for InputEventMouseMotion.pen_inverted

This commit adds support for stylus erasers, both for drawing and
choosing tools.  This may be supported on some styli by inverting them
as the property name suggests, or by holding a button while drawing with
the nib.

* Formatting fixes

src/Autoload/Tools.gd

@@ -18,6 +18,7 @@ var pen_pressure := 1.0
 var pen_pressure_min := 0.2
 var pen_pressure_max := 0.8
 var pressure_buf := [0, 0]  # past pressure value buffer
+var pen_inverted := false
 var mouse_velocity := 1.0
 var mouse_velocity_min_thres := 0.2
 var mouse_velocity_max_thres := 0.8
@@ -483,16 +484,26 @@ func handle_draw(position: Vector2i, event: InputEvent) -> void:
 	if Global.mirror_view:
 		draw_pos.x = Global.current_project.size.x - position.x - 1
 
-	if event.is_action_pressed("activate_left_tool") and _active_button == -1:
+	if event.is_action_pressed("activate_left_tool") and _active_button == -1 and not pen_inverted:
 		_active_button = MOUSE_BUTTON_LEFT
 		_slots[_active_button].tool_node.draw_start(draw_pos)
 	elif event.is_action_released("activate_left_tool") and _active_button == MOUSE_BUTTON_LEFT:
 		_slots[_active_button].tool_node.draw_end(draw_pos)
 		_active_button = -1
-	elif event.is_action_pressed("activate_right_tool") and _active_button == -1:
+	elif (
+		(
+			event.is_action_pressed("activate_right_tool")
+			and _active_button == -1
+			and not pen_inverted
+		)
+		or (event.is_action_pressed("activate_left_tool") and _active_button == -1 and pen_inverted)
+	):
 		_active_button = MOUSE_BUTTON_RIGHT
 		_slots[_active_button].tool_node.draw_start(draw_pos)
-	elif event.is_action_released("activate_right_tool") and _active_button == MOUSE_BUTTON_RIGHT:
+	elif (
+		(event.is_action_released("activate_right_tool") and _active_button == MOUSE_BUTTON_RIGHT)
+		or (event.is_action_released("activate_left_tool") and _active_button == MOUSE_BUTTON_RIGHT)
+	):
 		_slots[_active_button].tool_node.draw_end(draw_pos)
 		_active_button = -1
 
@@ -510,6 +521,8 @@ func handle_draw(position: Vector2i, event: InputEvent) -> void:
 		pen_pressure = remap(pen_pressure, pen_pressure_min, pen_pressure_max, 0.0, 1.0)
 		pen_pressure = clampf(pen_pressure, 0.0, 1.0)
 
+		pen_inverted = event.pen_inverted
+
 		mouse_velocity = event.velocity.length() / mouse_velocity_max
 		mouse_velocity = remap(
 			mouse_velocity, mouse_velocity_min_thres, mouse_velocity_max_thres, 0.0, 1.0

src/UI/ToolsPanel/ToolButtons.gd

@@ -1,10 +1,13 @@
 extends FlowContainer
 
+var pen_inverted := false
+
 
 func _input(event: InputEvent) -> void:
-	if not Global.has_focus or not Global.can_draw:
-		return
 	if event is InputEventMouseMotion:
+		pen_inverted = event.pen_inverted
+		return
+	if not Global.has_focus or not Global.can_draw:
 		return
 	for action in ["undo", "redo"]:
 		if event.is_action_pressed(action):
@@ -29,6 +32,13 @@ func _input(event: InputEvent) -> void:
 func _on_Tool_pressed(tool_pressed: BaseButton) -> void:
 	var button := -1
 	button = MOUSE_BUTTON_LEFT if Input.is_action_just_released("left_mouse") else button
-	button = MOUSE_BUTTON_RIGHT if Input.is_action_just_released("right_mouse") else button
+	button = (
+		MOUSE_BUTTON_RIGHT
+		if (
+			Input.is_action_just_released("right_mouse")
+			or (pen_inverted and Input.is_action_just_released("left_mouse"))
+		)
+		else button
+	)
 	if button != -1:
 		Tools.assign_tool(tool_pressed.name, button)