Improved HDR support

[walbourn]

The DirectXTex library can read & write floating-point format DDS files, but the only 'source' HDR format it supports is JPEG XR / HD-Photo.

The library should have support for another HDR format, such as OpenEXR--see here and here--, and functions for converting between the various color spaces:

• Linear RGB
• Rec 709 color primaries / sRGB
• Rec 2020 color primaries
• scRGB
• Adobe RGB
• Adobe Wide Gamut RGB

[walbourn]

Opened issues for HDR and PFM file formats which would be easy to implement directly in the library without having to rely on a 3rd party library for more HDR support. This along with PPM would round out the file format support to match the old D3DX library support.

[walbourn]

DirectXTex now supports HDR (Radiance RGBE)

[walbourn]

The colorspace conversions can now be implemented using TransformImage, so they don't need to be 'baked' into the library.

[walbourn]

Library now includes optional OpenEXR format support. See Adding OpenEXR