You can use the Q3View_Sync function (link) to ensure that a drawing operation has finished. You should call Q3View_Sync only after you've called Q3View_EndRendering .
You can use the Q3View_Flush function (link) to flush any image buffers maintained internally by a renderer. You should call Q3View_Flush only between calls to the Q3View_StartRendering and Q3View_EndRendering functions.