wayland: Fix SurfaceInterface::bufferSourceBox() when the viewport source rect is unset

Vscode starts with a valid source rect but it resets it eventually by
calling wp_viewport.set_source(-1, -1, -1, -1).

On the other hand, `current.viewport.sourceGeometryIsSet` would be true
if that happens.

SurfaceInterface::bufferSourceBox() should use the buffer size when the
source rect has been unset otherwise the damage handling code will be
confused and it's likely that the scene won't issue repaint requests.

src/wayland/surface_interface.cpp

@@ -787,7 +787,7 @@ QRegion SurfaceInterface::input() const
 
 QRectF SurfaceInterface::bufferSourceBox() const
 {
-    if (!d->current.viewport.sourceGeometryIsSet) {
+    if (!d->current.viewport.sourceGeometry.isValid()) {
         return QRectF(0, 0, d->bufferSize.width(), d->bufferSize.height());
     }