Fix background clearing when exposing with non-double buffered widgets

Fix background clearing when exposing with non-double buffered widgets

We used to set a flushed boolean whenever we flushing double buffered
areas to the window due to a non-db draw. We then read back from the
window if this was set. This broke when we were doing multiple paints
of the same area after a flush as we were re-reading the window each
time, overdrawing what was previously draw.