The main change is that the numFramesToSkipMetalRenderer member has been
removed. The old mechanism had some problems in AU plugins in Logic.
Specifically, when opening a temporary window, repaints would be ignored
until the numFramesToSkipMetalRenderer counter had been decremented to
zero. In practice, this meant that a PopupMenu might not repaint until
the highlighted item had changed five times or more.

With this change applied, repaints that were requested for a particular
window size will use the metal renderer as long as the window hasn't
changed size in the meantime.