It probably is easier to write their own rendering engine because, sadly, they've already have done it - for quite some time now, instead of using any native dr

It probably is easier to write their own rendering engine because, sadly, they've already have done it - for quite some time now, instead of using any native drawing functionality Qt renders the entire window contents in a pixel buffer and dumps that to a native toplevel window (FWIW GTK+ 3 and later do the same thing). In this case all they have to do is dump the pixel buffer to the canvas instead.