Blender Git Loki

Cycles: Support half and half4 textures.

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.