pushCrossFade
Snapshots a cross-fade operation between two images with the given @progress.
Until the first call to method@Gtk.Snapshot.pop, the start image will be snapshot. After that call, the end image will be recorded until the second call to method@Gtk.Snapshot.pop.
Calling this function requires two subsequent calls to method@Gtk.Snapshot.pop.
Parameters
progress
progress between 0.0 and 1.0