Move the front.offset setting above a check to shortcut reallocating buffers if

Move the front.offset setting above a check to shortcut reallocating buffers if
width and height stayed the same.  Rely on front.offset pointing at the origin
in the window (rather than the origin of the front buffer), and fix the coords
in swapbuffers, which were wrong.  This is different from other drivers because
we've got a back buffer with a 0,0 origin at the window's origin, which makes it
more convenient to have the front buffer match it.