Flip CZML pixel offset origin for billboard and label.

Because CZML is not yet versioned, we can't distinguish new from old CZML.  So, until we implement all our breaking changes in CZML 1.0, keep the existing convention in CZML.
Properly handle flipping sampled pixel offset values.
Merge pull request #1874 from AnalyticalGraphicsInc/flipCzmlPixelOffset

Flip CZML pixel offset origin for billboard and label.