BUG: When the viewport is defined between not round pixels, the size of the output imagedata and the size of the window's buffer could be different.
This is because the size of the 2 buffers are calculated differently:
int( Viewport[2]*winsize[0]+0.5)-1 - int(Viewport[0]*winsize[0])+1 is different from int(winsize[0]*Viewport[2]-Viewport[0])+0.5)
The result of that bug was that for some viewport, the output image was shifted in x. (the grabed buffer was larger than the output data).