QNX: fix race condition when clearing transparent buffer

Block flush until all bits have been flushed. This prevents Qt from trying to
draw over the buffer while it is still being cleared.

Change-Id: I49b90a7653ec3768411a1a94837bb31fec4d44e8
Reviewed-by: Frank Osterfeld <frank.osterfeld@kdab.com>
Reviewed-by: Andreas Holzammer <andreas.holzammer@kdab.com>
Reviewed-by: Bernd Weimer <bweimer@blackberry.com>

src/plugins/platforms/qnx/qqnxrasterbackingstore.cpp

@@ -185,8 +185,8 @@ void QQnxRasterBackingStore::beginPaint(const QRegion &region)
                                             platformWindow()->renderBuffer().nativeBuffer(), bg),
                                 "failed to clear transparent regions");
         }
-        Q_SCREEN_CHECKERROR(screen_flush_blits(platformWindow()->screen()->nativeContext(), 0),
-                            "failed to flush blits");
+        Q_SCREEN_CHECKERROR(screen_flush_blits(platformWindow()->screen()->nativeContext(),
+                    SCREEN_WAIT_IDLE), "failed to flush blits");
     }
 }