手机端地形显示问题terrain

**maptalks's version and what browser you use?**
----v1.0.0-rc.30 gl-layers v0.31.9

**Issue description**
----在微信端或是手机浏览器打开加载地形的案例，地形无法正常显示（无法正常拉伸出高度），请问大神是不是不支持手机端的地形渲染？

**Please provide a reproduction URL (on any jsfiddle like site)**
----http://examples.maptalks.com/examples/cn/3d/terrain/load

Start by reproducing the terrain example at http://examples.maptalks.com/examples/cn/3d/terrain/load in WeChat or a mobile browser, then compare it with a desktop browser using maptalks v1.0.0-rc.30 and gl-layers v0.31.9. Trace the terrain rendering path involved in that example and determine whether mobile rendering fails or is unsupported; done means the behavior is fixed or the supported-platform limitation is clearly documented.

Written by the indexing model from the issue text.