patch provided by Alfredo de Gref.
this adds radiance HDR image file support. So now at least we can save the fbuf (4x32 bits float colors) in Blender.
It doesnt change anything for internal support in imbuf for floa colors, so when reading, HDR files it still converts it todo 32 bits RGBA.
As an extra ive added that saving images with f3 now a los adds the optional extensión, when the f10 extensiones option is set.
One important note, i dont know the proper license for the code, it was provided without. Will await feedbak from Alfredo about it. For now ive added the standard Blender GPL Header.
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