Multithreading changes and more bugfixes

What's new:

• Faster sorting of render surfaces.
• New console functions: isVariable() and isCommand().
• No more HDR artifacts caused by the vignette.
• HDR calculation now performs with 4 frames delay.
• "physics_multithreaded" renamed to "physics_num_threads".
• All thread controls moved to EngineThreads, node update can work in multithreading mode, this can get a performance boost for particle systems.
• Number of supported callback arguments was increased to 8.
• Log file name can be specified by new "-engine_log" CLI option.
• Preview works only for images smaller than 4 Mb.
• Shaders has unique id's for easy sorting.
• Fixed selection of objects under terrain holes.
• Terrain holes disappear starting only from 4th LOD.
• Shadows from grass bushes.
• Support of GL_ARB_draw_instanced extension for better ATI compatibility in addition to existing GL_EXT_draw_instanced.
• No more stack errors while calling UnigineScript functions from the C++ API.
• Fixed major stack bug in UnigineScript.
• Update of benchmarking scripts.
• More tutorials on terrain creation.

We have also successfully integrated a third-party networking solution with Unigine, now we are performing different performance tests to ensure that it was a right choice of the technology. If the testing will be successfull, our customers have a chance to get networking ready out-of-the box even sooner than it was expected.