Call Yield() to unblock the main thread when refreshing.

Also, we don't seem to need this solar mutex locking and checking for
needs repaint flag here anymore, and not having these make the streaming
much faster esp on Windows.

Change-Id: I6e8ae82e5d986492ac576d28f11e2afffe954bc2
(cherry picked from commit c3bd435387ffbc6ae3c5a4861ac419d0a7092807)