Do the QML based painting when requested inside the UI thread, and
only copy the final canvas pixmap in the renderer thread (in paint()).
To avoid excessive re-drawing delay the painting via a timer to the
next event loop run.

The previous logic was still from Qt Quick 1 times, where the paint()
method was called in the GUI thread. This has changed in Qt Quick 2:
Here the paint() method will be called (on Unix) in a separate renderer
thread, making the drawRegion / onDrawRegion connections implicitly
deferred.

Change-Id: I298547013658e4cd1e94115305577bdb96cd0b2a
Reviewed-by: Ulf Hermann <ulf.hermann@digia.com>
Reviewed-by: Kai Koehne <kai.koehne@digia.com>