Rename framebuffer to _framebuffer and add getter
Rename framebuffer to _framebuffer and add getter

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