Fix viewport offset between webgl and css rendered content

[speigg]

There is a slight downwards shift in webgl rendered content along the viewport plane in the portrait axis. Not sure why (yet), but the shift is visible when both of the directions samples are loaded (webgl and css) and zoomed out. (It’s a little less obvious with the text from the webgl sample, but the text is supposed to be centered on the same directions as the circles from the css sample, and it’s slightly off).