layout(
dev_w,dev_h,cw,ch) ->k,dx,dy
Compute the integer-upscale layout of a canvas on a display mode
Returns the largest integer scale factor k such that cw*k x ch*k fits
inside dev_w x dev_h, plus the 0-indexed offsets dx, dy that center the
scaled image. Returns nil, err when the mode is smaller than the canvas
(k < 1).