Shared WebGL context

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/