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 k at which a cw x ch canvas fits a
dev_w x dev_h mode. Also returns the 0-indexed offsets dx, dy that
centre it. Returns nil and an error when the mode is smaller than the
canvas; the window's layout (gfx.x11) crops instead.