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/app.go

@@ -61,6 +61,10 @@ type FrameEvent struct {
 type ViewEvent interface {
 	implementsViewEvent()
 	ImplementsEvent()
+	// Valid will return true when the ViewEvent does contains valid handles.
+	// If a window receives an invalid ViewEvent, it should deinitialize any
+	// state referring to handles from a previous ViewEvent.
+	Valid() bool
 }
 
 // Insets is the space taken up by