wl: do not set to null the surface and target at touch_on_up()

On a multitouch screen, when several fingers are on the screen at
the same time, if we set to null surface and target on touch_on_up()
then we stop all the ongoing touch gestures and not only the one
of the finger that caused the up event. That is becasue
touch_on_motion() will refuse to process further events if surface
or target are null.

Instead of doing that do not set to null this pointers on the
touch_on_up() event. The value of those will be set again on
the next touch_on_down() event in any case.

Also make seat->touch_target a weak reference, that way if the
CogViewport gets destroyed before the touch-move and/or touch-up
events arrive it will be set to NULL automatically and we avoid
use-after-free of a stale pointer.

Related: #698
Related: https://bugs.webkit.org/show_bug.cgi?id=270516

platform/wayland/cog-platform-wl.c

@@ -768,7 +768,7 @@ touch_on_down(void              *data,
 
     CogWlViewport *viewport = wl_surface_get_user_data(
         surface); // Viewport is attached the context of the wl_surface. Check cog_wl_viewport_create_window();
-    seat->touch_target = viewport;
+    g_set_weak_pointer(&seat->touch_target, viewport);
     seat->touch.serial = serial;
     seat->touch.surface = surface;
 
@@ -820,13 +820,7 @@ touch_on_up(void *data, struct wl_touch *touch, uint32_t serial, uint32_t time,
         return;
     }
 
-    g_assert(seat->touch_target);
-    CogWlViewport *viewport = COG_WL_VIEWPORT(seat->touch_target);
-
-    struct wl_surface *target_surface = seat->touch.surface;
-    seat->touch_target = NULL;
     seat->touch.serial = serial;
-    seat->touch.surface = NULL;
 
     if (id < 0 || id >= 10)
         return;
@@ -838,6 +832,7 @@ touch_on_up(void *data, struct wl_touch *touch, uint32_t serial, uint32_t time,
     CogWlPlatform *platform = (CogWlPlatform *) cog_platform_get();
     CogWlPopup    *popup = platform->popup;
 
+    struct wl_surface *target_surface = seat->touch.surface;
     if (popup && popup->wl_surface) {
         if (target_surface == popup->wl_surface) {
             cog_popup_menu_handle_event(popup->popup_menu, COG_POPUP_MENU_EVENT_STATE_RELEASED, raw_event.x,
@@ -853,7 +848,8 @@ touch_on_up(void *data, struct wl_touch *touch, uint32_t serial, uint32_t time,
 
     struct wpe_input_touch_event event = {seat->touch.points, 10, raw_event.type, raw_event.id, raw_event.time};
 
-    CogView *view = cog_viewport_get_visible_view((CogViewport *) viewport);
+    CogWlViewport *viewport = COG_WL_VIEWPORT(seat->touch_target);
+    CogView       *view = cog_viewport_get_visible_view((CogViewport *) viewport);
     if (view)
         wpe_view_backend_dispatch_touch_event(cog_view_get_backend(view), &event);