Change semantics of UpdateRequest events created by QWindow::requestUpdate()

Currently this type of UpdateRequest forces a repaint, e.g. for widgets user paintEvent code is always called, even if nothing is marked dirty.

This is perhaps not that useful in the general case. Instead we should make QWindow UpdateRequest behavior match QWidget UpdateRequest:

• The UpdateRequest event is the correct place to repaint any dirty areas.

This mostly applies to Raster, OpenGL would do a full repaint anyway (unless there is some sort of FBO caching setup.)

There might be cases where a full repaint is needed, but this can be accomplished by using a flag and/or the Expose event.

The alternative to this would be creating a new type of "update" event - an unfortunate duplication.