IBL Skybox is overexposed

Currently skyboxes are over exposed.  That is because the source data for the skybox is usually an HDR image where the color values are outside of the 0.0 - 1.0 range.  These colors need to be tonemapped (they are) and account for exposure.

The current result is that areas of the skybox that are above 1.0 are clipped, and large parts of some images will just end up being white.  This leads to things like the sky not being blue in bright sunny images.