[REG]: When resizing a QQuickWindow that has some heavy painting going on then it will flicker from white to the painted view

When resizing a QQuickWindow that has some heavy painting going on then it will flicker from white to the painted view. To reproduce this add the line:

usleep(800);

to after the line

glDrawArrays(GL_TRIANGE_STRIP, 0, 4);

inside the openglunderqml example. Then resize from the left hand corner (for some reason it does not happen from the right hand side).

In testing here it seems to be working fine on Windows but not on Mac. This is a regression against Qt 5.1.1