`emission_luminance` soft-max should be higher than its default

[portsmouth]

In https://github.com/AcademySoftwareFoundation/OpenPBR/pull/231 we added emission_weight. As part of that change, we made the emission_luminance default to 1000 nits. We didn't alter the soft-max from 1000 nits though.

This means that in the UI, the values will range over [0, 1000] with the slider at the far right end of the range. That could be a bit inconvenient perhaps, as the user will have to type in a higher value to increase the luminance past the default. I'd suggest instead we make the soft-max be e.g. 2000 nits, so the slider defaults to the middle of the range.