Handle wayland touch cancel message

Suppose host has some three-finger gesture. Then we get the following sequence
of events:
DOWN-DOWN-DOWN-MOTION-CANCEL

Note that there is no UP in this sequence. So if we don't handle CANCEL then
we end up thinking that fingers are still touching the screen. Ideally we
should inform the application that cancel has happened as not to trigger
spurious taps but still this is way better than being stuck with phantom
finger touch.

src/video/wayland/SDL_waylandevents.c

@@ -932,6 +932,28 @@ static void touch_handler_frame(void *data, struct wl_touch *touch)
 
 static void touch_handler_cancel(void *data, struct wl_touch *touch)
 {
+	struct SDL_WaylandTouchPoint *tp;
+	while ((tp = touch_points.head)) {
+		wl_fixed_t fx = 0, fy = 0;
+		struct wl_surface *surface = NULL;
+		int id = tp->id;
+
+		touch_del(id, &fx, &fy, &surface);
+
+		if (surface) {
+			SDL_WindowData *window_data = (SDL_WindowData *)wl_surface_get_user_data(surface);
+
+			if (window_data) {
+				const double dblx = wl_fixed_to_double(fx) * window_data->pointer_scale_x;
+				const double dbly = wl_fixed_to_double(fy) * window_data->pointer_scale_y;
+				const float x = dblx / window_data->sdlwindow->w;
+				const float y = dbly / window_data->sdlwindow->h;
+
+				SDL_SendTouch((SDL_TouchID)(intptr_t)touch, (SDL_FingerID)id,
+					      window_data->sdlwindow, SDL_FALSE, x, y, 1.0f);
+			}
+		}
+	}
 }
 
 static const struct wl_touch_listener touch_listener = {