This is an initial commit for half texture support in Cycles.
It adds the basic infrastructure inside of the ImageManager and support for these textures on CPU.
Supported:
* Half Float OpenEXR images (can be used for e.g HDRs or Normalmaps) now use 1/2 the memory, when loaded via disk (OIIO).
ToDo:
Various things like support for inbuilt half textures, GPU... will come later, step by step.
Part of my GSoC 2016.