#154 White-out on all operators except Mantuik '08

[J Bytheway]

With several exr images created by hugin_hdrmerge all tone-mapping operators except Mantuik '08 yield a pure-white output. An example is attached (heavily cropped to be small enough to be an attachment). I suspect this is related to the region in the top right of this image whcih appears black in the Mantuik '08 version. This was overexposed in all the images which went into the merge. I guess the pixel values there are -inf or NaN or something, which is causing problems.

Apart from the tone-mapping failing, the preview is also unhelpful, showing as all-black. Also, the band around the histogram above the image cannot be dragged at its lower end. I think that at least should be fixed, even if making more of the tone-mapping operators work in this context is deemed inappropriate.

I've tried both version 2.3.0 and git commit 7025aef2 with the same results.

[J Bytheway]

Image demonstrating problem