app: add cross-platform empty view event detection

Custom rendering applications need to be prepared to handle empty view events,
as an empty view event is sent during window shutdown. However, the current
implementation requires applications to write a platform-specific helper
function for each supported platform in order to check whether a received
view event is empty. This commit provides a safe, convenient, cross-platform
method that applications can use to detect this special view event and respond
to it.

Signed-off-by: Chris Waldon <christopher.waldon.dev@gmail.com>

app/os_unix.go

@@ -21,6 +21,9 @@ type X11ViewEvent struct {
 
 func (X11ViewEvent) implementsViewEvent() {}
 func (X11ViewEvent) ImplementsEvent()     {}
+func (x X11ViewEvent) Valid() bool {
+	return x != (X11ViewEvent{})
+}
 
 type WaylandViewEvent struct {
 	// Display is the *wl_display returned by wl_display_connect.
@@ -31,6 +34,9 @@ type WaylandViewEvent struct {
 
 func (WaylandViewEvent) implementsViewEvent() {}
 func (WaylandViewEvent) ImplementsEvent()     {}
+func (w WaylandViewEvent) Valid() bool {
+	return w != (WaylandViewEvent{})
+}
 
 func osMain() {
 	select {}