Question about support for Multisampled Renderbuffers in THREE 101

[gchoqueux]

First, the support for Multisampled Renderbuffers in THREE 101, it's only WebGL 2.0.

Use this feature in iTowns, so the Renderbuffers can be used to calculate the depth buffer and color buffer with antialiasing. We would avoid computing this depth buffer in our shaders. It's still necessary to re-check if it's not possible to recover this depth buffer following renderer.render(). The other problem is that extention gl_FragDepthEXT isn't known in WebGL 2.0 so logarithmic Depth Buffer isn't supported. We must find how to do it in WebGL 2.0 or no longer use this solution.