Fixed Label.getScreenSpaceBoundingBox by using the glyph dimensions instead of the billboard dimensions.  Using the billboard dimensions assumes that the text is very tightly centered in the glyph itself which isn't always the case with the SDF generated text.  We'll look to tighten up the glyph size so it isn't unnecessarily big, but this this change should work in all cases so matter how much padding we have around the glyphs.
Fixed Label.getScreenSpaceBoundingBox by using the glyph dimensions
instead of the billboard dimensions.  Using the billboard dimensions
assumes that the text is very tightly centered in the glyph itself which
isn't always the case with the SDF generated text.  We'll look to
tighten up the glyph size so it isn't unnecessarily big, but this this
change should work in all cases so matter how much padding we have
around the glyphs.

YonatanKra/cesium,CesiumGS/cesium,YonatanKra/cesium,likangning93/cesium,YonatanKra/cesium,progsung/cesium,progsung/cesium,likangning93/cesium,CesiumGS/cesium,jasonbeverage/cesium,AnalyticalGraphicsInc/cesium,CesiumGS/cesium,CesiumGS/cesium,YonatanKra/cesium,AnalyticalGraphicsInc/cesium,likangning93/cesium,jasonbeverage/cesium,likangning93/cesium,likangning93/cesium,CesiumGS/cesium