add multithreading to runtime converter

[unitycoder]

could do calculations in another thread, return byte array, apply that to texture

for example in this script, could start simple thread for each task:
NormalMapTools.FilterMedian()
NormalMapTools.CreateNormalmap()
NormalMapTools.CreateSpecular()
then wait for results, apply to texture.
https://github.com/unitycoder/NormalMapFromTexture/blob/master/Assets/Extensions/unitycoder_com/DemoRunTime/Scripts/NormalMapMakerRuntimeDemo.cs