HDR Texture Sampling (pt 2)

I had proposed a patch for HDR texture sampling a ways back, and Brecht has committed it (with a few small modifications).  I feel like there's a texture slot bug lurking in there that I may need to fix, but I'll have to verify that.

Essentially, the change allocates 5 of the 100 texture slots to be full floating point textures.  Generally the data set there will be linear (and Brecht changed the monikers for those to be "color" vs. "non-color" instead of linear vs sRGB).  Any source texture that is naturally a float format, such as EXR or 32-bit float TIFF, or HDR, will automatically get one of those slots.

A couple of things to keep in mind:

1. You've only got 5 full-float texture slots, so be judicious in your use of them.  Any textures you don't want using the those slots need to be saved in a format that is not floating point on disk, and they will land in the regular texture slots instead.
2. Anyone who was using the 100 slots before for non-float textures, though, is in for a surprise; they've lost 5 regular (non-float) texture slots.  Never fear, all you have to do is change some textures to a float format (such as EXR) and you'll get back to the 100 total slots that way.

The management is a little bit manual for now, but the long-term strategy for Cycles I think is that texture management will be done in a different way.  Textures will be packed into layered versions, and the sampling of them will occur using a more manual sampling (that is slower but higher quality).  This way the 100 texture limit will be eliminated, and better sampling like anisotropic kernels can be used, even on GPU.