cesium配置兼容win7 低版本浏览器

[klren0312]

https://blog.csdn.net/Zk9509/article/details/132076491

[klren0312]

contextOptions: {
         // cesium状态下允许canvas转图片convertToImage
        webgl: {
          alpha: true,
          depth: false,
          stencil: true,
          antialias: true,
          premultipliedAlpha: true,
          preserveDrawingBuffer: true,//通过canvas.toDataURL()实现截图需要将该项设置为true
          failIfMajorPerformanceCaveat: false, 
        },
        allowTextureFilterAnisotropic: true,
        requestWebgl1: true, //设置使用Webgl1
     }