e3b2d11dba
Due to changes in the renderer in fc560d4 the engine was created once a
widget is realized, not when the widget is created. If a Flutter
application changed the default my_application.cc template to show the
Flutter widget after plugins are run then these plugins would not be
able to access the engine.
Solved by removing the GdkWindow from the renderer constructor and
setting in later when the widget is realized. This works because the
renderer is not used until the widget is realized.
Fixes https://github.com/flutter/flutter/issues/144873