fix voxel gi

[e2002e]

This is a patch for the voxels module:

• Fix occlusion being multiplied by voxels specular instead of material's specular.
• Implements voxels shadows and also voxels refraction. Deals with material only illumination.
• Fix screen space refraction depth test that made background only appear through transparent objects.
• Removed the normalization of viewPos in the use of reflect / refract.
• Compute the environment light in the voxelizer and pass it in the "voxels" image for accuracy.

TODO:

• Voxels refraction only work with screen space refraction on, yet no hint is given in the UI.

Bugs:

• Forward renderer has wrong coordinates for sampling the voxels resolving images, so it results in strange projections.
  The materials created with make_forward_base.py are working fine in deferred.

[e2002e]

@lubos, you can merge than if you don't mind voxels not working in forward renderer.
Everything is functional in deferred,