This doesn't work efficiently for non-square arrays.

E.g. if you have a 128x32 then you don't want to interleave the y bits once they are exhausted (only need 5 bits of y, but 7 bits of x to fully address this).

The posted implementation would produce

    y6,x6,y5,x5,y4,x4,y3,x3,y2,x2,y1,x1,y0,x0

but really we want:

    x6,x5,y4,x4,y3,x3,y2,x2,y1,x1,y0,x0