function computeScreenAwareSize(cssPx) {
        var screenPx = window.devicePixelRatio * cssPx;
        return Math.max(1, Math.floor(screenPx)) / window.devicePixelRatio;
    }
Find a value usable for a dom node size such that the likelihood that it would be
displayed with constant screen pixels size is as high as possible.

e.g. We would desire for the cursors to be 2px (CSS px) wide. Under a devicePixelRatio
of 1.25, the cursor will be 2.5 screen pixels wide. Depending on how the dom node aligns/"snaps"
with the screen pixels, it will sometimes be rendered with 2 screen pixels, and sometimes with 3 screen pixels.