Min/max need to support negative values for signed datatypes

[normanrz]

Webknossos supports signed datatypes such as float. However, the min/max settings below the histogram don't accept negative values.

[normanrz]

@philippotto Does the rendering support negative float values already?

[philippotto]

I'm not sure. Do you happen to have a DS layer with negative floats to that I could test it out?

[philippotto]

I think, the rendering itself should work fine wiht negative floats.