consistent font line-spacing for Label
consistent font line-spacing for Label

changes multi-line Label line-spacing basis from per-label max-glyph-height to font-size

likangning93/cesium,progsung/cesium,AnalyticalGraphicsInc/cesium,CesiumGS/cesium,CesiumGS/cesium,progsung/cesium,CesiumGS/cesium,likangning93/cesium,likangning93/cesium,likangning93/cesium,AnalyticalGraphicsInc/cesium,likangning93/cesium,CesiumGS/cesium,CesiumGS/cesium