[depth] rewrite C module, "class" ZBufferTo3D

Allowing multi depth-camera in the same simulation. Former
globals are now wrapped in the Python ZBufferTo3D "class" (C
PyZBufferTo3D struct).
[profiler] do not record time if not profiling
[builder] add stereo mode and eye separation

see
http://www.blender.org/documentation/blender_python_api_2_65_release/bpy.types.SceneGameData.html#bpy.types.SceneGameData.stereo_eye_separation