GeometryExporter: Considering the engine doesn’t support quads anymore, the isTriangle code is now dead.

@algrs ok, I think I managed to get it to work now. I’m keeping a vertices array to map indices per object.

The WebGlRenderer.render's forceClear argument forces a
clear -only- for buffers with their .autoClear property is set to true.

Took me a while to find a bug in my code as I didn't understand
these flags overode the forceClear parameter for WebGlRenderer.render

Merge branch 'dev' of https://github.com/mrdoob/three.js; branch 'dev' of https://github.com/benbro/three.js into dev