Purge remaining use of "up" and "down" in the specification

Purge remaining use of "up" and "down" (resp. "top" and "bottom") in the specification. It is just confusing. Only time it is invoked should be the swapchain note, where "up" and "down" is relative to the monitor as it stands on the desk. Otherwise there is no up and down; only increasing and decreasing coordinates.

E.g.

> x and y are the viewport’s upper left corner (x,y).

There's no upper left corner if the framebuffer is offscreen. Should instead say something like "x and y represent the viewport origin in image coordinates".