Unable to drag shapes over the antimeridian on 2D globe

[zglueck]

Use the DraggingShapes example and switch to flat globe. Attempt to drag shape across the antemeridian, shape will stop on the antemeridian.

[zglueck]

It does work using the WorldWind.ABSOLUTE altitude mode. Perhaps this is a intersection calculation issue with terrain in a flat globe.

[zglueck]

Noticed the offset coordinate determination in computeScreenOffsetFromReferencePosition used a pattern that would break in certain cases, specifically when an altitude mode other than absolute was used and the click offset to the reference position spanned the antimeridian. Correcting to match the pattern used in 8c127c1.