Changed WebGLRenderer's sortFacesByMaterial to only split geometry if MeshFaceMaterial is used at the first render.

Fixes #2638

Before attributes were enabled just once, when creating a new program and then they were never disabled (with exception of UVs).

Now before rendering of the object existing enabled attributes are disabled and attributes required by program are enabled (these GL state changes are cached so they happen only when active geometry or material changes inside a frame).

There is still some gray zone for more exotic rendering methods (immediate render objects and render plugins do not disable outstanding enabled attributes, they just enable attributes they need) but we'll address these if they start to make problems.

Performance seems to be ok, at least for current examples, though there may be some use cases which could get dramatically slower with these changes (before optimizations naive attribute disabling / enabling every time halved the framerate in performance stress test).

Fixes #2633 (hope so)

This allows to work around removed CubeGeometry sides functionality.

Now we can finally mess with car materials directly in the scene file.

Also, removed webgl_trackballcamera_earth as there is already a example showing the usage and another one using this scene.

Also, removed GeometryLoader. I'll try doing the JSONLoader to GeometryLoader change again in the future.

@alteredq: I'll try to tackle Minecraft AO next.

See #2623.