Allow canceling drag-and-drop with right mouse button

This is a small usability enhancement, that allows users to cancel
drag-and-drop without the need to press the escape key on the keyboard.

scene/main/viewport.cpp

@@ -1742,6 +1742,11 @@ void Viewport::_gui_input_event(Ref<InputEvent> p_event) {
 	if (mb.is_valid()) {
 		Point2 mpos = mb->get_position();
 		if (mb->is_pressed()) {
+			if (gui.dragging && mb->get_button_index() == MouseButton::RIGHT) {
+				_perform_drop();
+				set_input_as_handled();
+				return;
+			}
 			MouseButtonMask button_mask = mouse_button_to_mask(mb->get_button_index());
 			if (!gui.mouse_focus_mask.is_empty() && !gui.mouse_focus_mask.has_flag(button_mask)) {
 				// Do not steal mouse focus and stuff while a focus mask without the current mouse button exists.

tests/scene/test_viewport.h

@@ -1341,8 +1341,8 @@ TEST_CASE("[SceneTree][Viewport] Controls and InputEvent handling") {
 				SEND_GUI_MOUSE_MOTION_EVENT(on_d, MouseButtonMask::NONE, Key::NONE);
 
 				// Force Drop doesn't get triggered by mouse Buttons other than LMB.
-				SEND_GUI_MOUSE_BUTTON_EVENT(on_d, MouseButton::RIGHT, MouseButtonMask::RIGHT, Key::NONE);
-				SEND_GUI_MOUSE_BUTTON_RELEASED_EVENT(on_a, MouseButton::RIGHT, MouseButtonMask::NONE, Key::NONE);
+				SEND_GUI_MOUSE_BUTTON_EVENT(on_d, MouseButton::MIDDLE, MouseButtonMask::MIDDLE, Key::NONE);
+				SEND_GUI_MOUSE_BUTTON_RELEASED_EVENT(on_a, MouseButton::MIDDLE, MouseButtonMask::NONE, Key::NONE);
 				CHECK(root->gui_is_dragging());
 
 				// Force Drop with LMB-Down.
@@ -1351,6 +1351,15 @@ TEST_CASE("[SceneTree][Viewport] Controls and InputEvent handling") {
 				CHECK(root->gui_is_drag_successful());
 
 				SEND_GUI_MOUSE_BUTTON_RELEASED_EVENT(on_d, MouseButton::LEFT, MouseButtonMask::NONE, Key::NONE);
+
+				node_a->force_drag(SNAME("Drag Data"), nullptr);
+				CHECK(root->gui_is_dragging());
+
+				// Cancel with RMB.
+				SEND_GUI_MOUSE_BUTTON_EVENT(on_d, MouseButton::RIGHT, MouseButtonMask::RIGHT, Key::NONE);
+				CHECK_FALSE(root->gui_is_dragging());
+				CHECK_FALSE(root->gui_is_drag_successful());
+				SEND_GUI_MOUSE_BUTTON_RELEASED_EVENT(on_a, MouseButton::RIGHT, MouseButtonMask::NONE, Key::NONE);
 			}
 		}