app: [API] refactor app.ViewEvent to support wayland

This commit changes the definition of app.ViewEvent on Unix systems
to be an interface implemented by two different concrete types:
X11ViewEvent and WaylandViewEvent. Each provides the necessary fields
for custom rendering on its display protocol.

This is a breaking change for all users of app.ViewEvent on
Linux/BSD, as it now is an interface instead of a concrete type.

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

app/os_unix.go

@@ -10,13 +10,33 @@ import (
 	"unsafe"
 )
 
-type ViewEvent struct {
+// ViewEvent provides handles to the underlying window objects for the
+// current display protocol.
+type ViewEvent interface {
+	implementsViewEvent()
+	ImplementsEvent()
+}
+
+type X11ViewEvent struct {
 	// Display is a pointer to the X11 Display created by XOpenDisplay.
 	Display unsafe.Pointer
 	// Window is the X11 window ID as returned by XCreateWindow.
 	Window uintptr
 }
 
+func (X11ViewEvent) implementsViewEvent() {}
+func (X11ViewEvent) ImplementsEvent()     {}
+
+type WaylandViewEvent struct {
+	// Display is the *wl_display returned by wl_display_connect.
+	Display unsafe.Pointer
+	// Surface is the *wl_surface returned by wl_compositor_create_surface.
+	Surface unsafe.Pointer
+}
+
+func (WaylandViewEvent) implementsViewEvent() {}
+func (WaylandViewEvent) ImplementsEvent()     {}
+
 func osMain() {
 	select {}
 }
@@ -46,5 +66,3 @@ func newWindow(window *callbacks, options []Option) error {
 	}
 	return errors.New("app: no window driver available")
 }
-
-func (_ ViewEvent) ImplementsEvent() {}