Shared WebGL context

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**maptalks's version and what browser you use?**
v0.43.0, chrome

**Issue description**
Layers do not share WebGL context, which creates glitches when a map has more than one 3d layer. For example a buildings layer with a 3D line will not hide the line behind the buildings.
In particular, I am implementing a 3D terrain layer but this layer will not hide (behind mountains) any other 3d element in the map, such as buildings.

**Please provide a reproduction URL (on any jsfiddle like site)**
The following example fraws two vector layers. Due to the WebGL context not being shared, the rendering is wrong.
[http://jsfiddle.net/h6z5jtky/2/](http://jsfiddle.net/h6z5jtky/2/)

Start with the linked JSFiddle reproduction and trace how the two vector layers create and use their WebGL contexts. Confirm the rendering behavior with a buildings layer and a 3D line or terrain layer; done means 3D elements are correctly occluded across layers, including lines behind buildings or mountains.

Written by the indexing model from the issue text.