make it easier to zero-out the transferfcn widget values
make it easier to zero-out the transferfcn widget values

The box shown for the widget is extended to essentially go "below zero",
so you can drag lines to be definitely 0, easily and then the value
is just clamped to 0.

MengjiaoH/ospray,MengjiaoH/ospray,MengjiaoH/ospray,ospray/OSPRay,MengjiaoH/ospray,ospray/OSPRay,ospray/OSPRay,wilsonCernWq/ospray,MengjiaoH/ospray,ospray/OSPRay,wilsonCernWq/ospray,wilsonCernWq/ospray