Restored old flushing behavior in -graphicssystem opengl on desktop.

Restored old flushing behavior in -graphicssystem opengl on desktop.

Change 284211ccbd2cbd recently introduced a fix for EGL, to prevent
flushing when nothing has been rendered into the back buffer. However,
the skip should only be done when there's no partial update support in
the window surface. If there is partial update support we can still
flush as usual.

Reviewed-by: Gunnar Sletta