ComputeScreenSpacePosition for polygon. Temporary fix for issue #746.

this code is similar to billboards and labels. Except for the fact it uses the first position from the polygon to compute the screen space position.

from @mramato, We should move to computeScreenSpaceBounds which is generic across all primitives and return a BoundingRectangle. For billboards, the rectangle would include the full bounds of the billboard and a center property would return the same value that computeScreenSpacePosition currently does.

This sounds like a good idea in the long term.